HCA415 Community & Public Health-WK2

Chapter 4 Epidemiology of Infectious Diseases

Learning Objectives

After reading this chapter, you should be able to:

Define the term infectious disease.

Explain the significance of infectious diseases for community health.

Describe modes of transmission of infectious diseases.

Apply the epidemiologic triangle to at least three infectious diseases.

State two methods for the prevention of infectious disease outbreaks.

4.1 Introduction

Infectious diseases are a major focus of epidemiology and community health. Globally, infectious and parasitic diseases account for a significant proportion of deaths. In 2008, the World Health Organization estimated that about 15% (8.5 million) of a total of 56.9 million deaths globally were associated with infectious and parasitic conditions. The leading infectious killers and their respective numbers of deaths in millions were tuberculosis (1.3), human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS) (1.8), and malaria (0.8). Moreover, childhood infectious diseases such as whooping cough and measles were among the leading causes of death among the world's children (WHO, 2011).

Photograph of an airport arrivals display.

Exactostock/SuperStock

Modern technological advances such as air travel could contribute to the quick, international spread of infectious disease.

Great strides have been made in the control of infectious diseases. By the mid-20th century, some public health experts believed that the tide of mortality and morbidity from infectious diseases, especially those caused by bacteria, had been reduced and that infectious diseases might even be eradicated by newly introduced antibiotics and other "magic bullets." Today, few public health experts hold this opinion, as microbes have developed resistance to antibiotics, causing the reemergence of once-controlled infectious diseases. Conditions known as "emerging infections" have gained a foothold worldwide and continue to endanger the population.

In the United States, infectious diseases continue to be major causes of mortality, although chronic noncommunicable diseases such as heart disease, cancer, and chronic respiratory disease form the top tier of leading causes of death. The category of pneumonia and influenza was the eighth leading cause in 2009 (CDC, 2013a).

The advent of an increasingly international world is one of the influences that could lead to epidemics of infectious diseases. Modern jet travel means that people can travel rapidly from one distant area to another. Nonendemic diseases, such as malaria, may be imported into the United States by travelers returning from areas in which these conditions are endemic. A dangerous communicable disease that has broken out in a remote corner of the globe can be introduced within a 24-hour period to a major city in Asia or Europe. From these continents, the disease can spread to the United States. As an extreme fictional example, the 2011 medical disaster film Contagion portrayed the epidemic spread of a lethal virus transmitted by fomites (inanimate objects such as skin or hair), attempts by epidemiologists and public health officials to identify and contain diseases, the loss of a social order due to the pandemic, and the introduction of a vaccine to halt the disease's spread.

Drug Resistant Bacteria Emerges

Leading health care professionals discuss the history of antibiotics, and the new resistant bacteria that have developed.

Critical Thinking Question:

How successful do you think the push to restrict antibiotic use has been? If not, what measures might still be taken?

Along with the trend of rapid international travel, major growth in international trade of food and medicines during the current century can also contribute to the spread of diseases. Improperly processed foods and medicines can bypass normal inspection and safety procedures, especially when the volume of such products exceeds the capacity of health authorities.

An additional possible factor linked with the spread of infectious diseases is deforestation in many parts of the world. Deforestation causes the destruction of natural ecosystems that are important for human survival. Among the consequences of ecosystem loss is the potential for increases in runoff of pollutants into human water supplies and changes in the distribution of disease-causing vectors. Growing urbanization and pressures from the human population explosion cause people to move into previously forested areas, where they may come into contact with new vectors and infectious disease agents.

This chapter provides an overview of infectious diseases and their impact on the health of communities. Topics covered include the discovery of disease-causing microbes, the epidemiologic triangle (agent, host, and environment), examples of significant infectious diseases, and methods for the prevention of communicable diseases.

4.2 Microbes in History

Chapter 1 described historical accounts of dramatic epidemics that caused devastating effects. Examples of historically significant epidemics were the Black Death (caused by Yersenia pestis), recurring smallpox epidemics, and pandemic influenza. The Black Death ravaged the populations of Europe, northern Africa, and the Middle East from 1346 to 1352 (McEvedy, 1988). In 1346, the Black Death killed 20 million people in Europe. The 1918–1919 Spanish flu pandemic was responsible for 50 million deaths worldwide (Taubenberger & Morens, 2006).

Before the late 19th century, the causes of infectious diseases were largely unknown. Gradually, during the mid-19th century, disease detectives became aware of microbes as causes of human diseases and developed the germ theory of disease. The germ theory of disease arose from the discovery that certain infectious diseases were caused by microorganisms that invade the body. Microorganisms are small organisms that cannot be seen without magnification. This germ theory of disease contributed to an understanding of the function that microbial agents perform in causing the spread of infectious disease. Eventually these new insights gained the attention of the public health community and led to the development of disease prevention efforts.

Among the scientists who stood out in history as instrumental in identifying causative microbes for infectious diseases were Ignaz Semmelweiss in 1840 and Robert Koch in 1847. Other breakthroughs in the control of infectious diseases included Edward Jenner's development of a vaccine against smallpox and Jonas Salk's formulation of a polio immunization in 1953. These innovations lead to the eradication of many formerly common epidemic diseases worldwide. Smallpox, the ancient scourge of humanity, was declared eradicated in 1979. Polio has a very limited presence in the United States, and does not exist in epidemic form in most parts of the globe.

Drawing of Ignaz Semmelweiss (1745–1822).

Silvio Fiore/SuperStock

Ignaz Semmelweiss (1745–1822) contributed to the germ theory of disease in 1840 with his theory that hand contamination might be responsible for the spread of disease. Semmelweiss's hypothesis led to hand washing and, therefore, a decreased spread of infection in Viennese hospitals.

Ignaz Semmelweiss

Ignaz Semmelweiss was a clinical assistant in obstetrics and gynecology at a Vienna hospital in the mid-19th century. According to Friis and Sellers (2009, p. 35),

In 1840, he noticed a much higher mortality rate of puerperal fever (childbirth fever) among the women who were located in the teaching quarters for medical students and physicians than in the teaching areas for midwives. He hypothesized that medical students and physicians had contaminated their hands during autopsies, and consequently transmitted infections while caring for women in the maternity wing. When the practice of hand washing with chlorinated solutions was introduced and practiced, the death rate for puerperal fever in the wards for medical students and physicians dropped to a rate equal to the wards for midwives.

Robert Koch

As discussed in earlier chapters, in the late 19th century, Robert Koch verified that a specific living organism caused a human disease. According to Friis and Sellers (2013), "Koch's postulates advanced the theory of specific disease agents" (p. 48) by explaining the prerequisites needed for establishing that a microorganism causes a particular disease. The following conditions must be met:

The microorganism must be observed in every case of the disease.

The microorganism must be isolated and grown in pure culture.

The pure culture must, when inoculated into a susceptible animal, reproduce the disease.

The microorganism must be observed in, and recovered from, the experimentally diseased animal. (King, 1952, p. 350–361)

Although Koch's postulates were relevant for bacterial diseases, such as tuberculosis, they were later shown inaccurate in regards todiseases caused by some viruses and conditions that have a complex causal factors (Porta, 2008).

Edward Jenner and Smallpox Vaccinations

In 1798, Edward Jenner was credited with the development of an effective smallpox vaccination. Jenner observed that cowpox, an infection caused by the vaccinia virus, afforded protection against smallpox. Dairymaids who developed cowpox from touching the pustules of infected cows' udders were immune to smallpox. He developed a smallpox vaccine by using scrapings from lesions on the skin of people infected with cowpox.

Smallpox is a serious, disfiguring, and contagious disease caused by the variola virus. Survivors of smallpox suffered from facial scars and blindness. During the 18th century in Europe, almost 30% of reported cases of blindness were attributed to smallpox. In 1898, nearly 100% of adolescents in Vietnam suffered from deeply pitted facial scars and nine-tenths of all cases of blindness were due to routine transmission of smallpox.

One of the great public health achievements was the global eradication of smallpox. In the 1950s, approximately 50 million cases of smallpox occurred annually worldwide. By 1967, the number dropped to 10 to 15 million following the World Health Organization's launch of a plan to eradicate smallpox. In 1979, smallpox was declared eradicated (WHO, 2010).

Efforts to Eradicate Polio

During the mid-20th century (from approximately the late 1940s to the early 1950s), polio crippled about 35,000 individuals every year in the United States and was one of the most feared diseases at that time in the United States (CDC, 2012a). The occurrence of polio causing paralysis peaked in 1952, when 21,000 cases were reported.

Polio is an incurable and potentially crippling infectious disease of the nervous system caused by a virus—the poliovirus. By far the majority of polio cases (approximately 95%) are asymptomatic; nevertheless, those with asymptomatic infections can transmit polio to other individuals. Somewhat fewer than 1% of polio infections result in paralysis; this form of polio can be fatal when the virus attacks the respiratory system. Because humans comprise the only known reservoir for the disease, eradication is possible through polio vaccination campaigns.

Two medical pioneers, Dr. Jonas Salk and Dr. Albert Sabin, created vaccines against polio. In the 1950s, while at the University of Pittsburgh, Dr. Salk developed an inactivated polio vaccine (IPV) that became known as the eponymous Salk Polio Vaccine. The Salk vaccine was made from a killed vaccine and received government approval in 1955 for public use. Inactive vaccines consist of virus particles that are grown in cultures and then killed with heat or formaldehyde. These virus vaccines do not reproduce, and booster shots are required periodically to reinforce the immune response.

Dr. Sabin formulated an oral polio vaccine (OPV) based on a live polio virus. Live or attenuated virus vaccines are created by reducing the virulence of a pathogen but still keeping it viable. Approval for public use was granted in 1963. Subsequent to the introduction of vaccines, the numbers of cases declined precipitously in the United States until 1979, a milestone for the last reported polio case caused by wild poliovirus. Currently, eradication efforts through targeted immunization programs are underway to free the world from the ravages of polio.

4.3 The Epidemiologic Triangle

How can we explain the occurrence of infectious diseases in the community? A model for the etiology of infectious diseases known as the epidemiologic triangle encompasses three major factors: agent, host, and environment. The triangle is helpful for explaining and controlling the occurrence of disease outbreaks caused by microbial agents. Settings where disease outbreaks may occur include the community in general as well as subunits within the community—schools, hospitals, and other institutional settings. Refer to Figure 4.1 for an illustration of the epidemiologic triangle.

Source: Adapted from Friis & Sellers, T. A. (2009). Epidemiology for public health practice (4th ed.; p. 439). Sudbury, MA: Jones and Bartlett Publishers.

The epidemiologic triangle shows the three components involved in the spread of infectious disease: the environment, agent, and host.

Agents of Infectious Disease

An infectious disease (communicable disease) is one caused by an infectious agent such as a bacterium, virus, parasite, or fungi. When accounting for the etiology of an infectious disease, the triangle specifies that a microbial agent must be present for an infection to occur. Examples of microbial agents include bacteria, viruses and rickettsia, fungi, protozoa, helminths, and arthropods. Infections with the human immunodeficiency virus (HIV), seasonal influenza, and Salmonella-associated food-borne disease outbreaks are all examples of conditions caused by infectious agents. Table 4.1 lists examples of infectious disease agents and diseases that they produce.

Table 4.1: List of infectious disease agents/vectors and the diseases that they produce

Disease Agents/Vectors	Diseases Produced
Bacteria	Tuberculosis, salmonellosis, streptococcal infections, and methicillin-resistant Staphylococcus aureus
Viruses	Viral hepatitis A, herpes simplex virus, influenza, and viral meningitis
Rickettsia	Q fever, Rocky Mountain spotted fever, and rickettsial pox
Fungi	San Joaquin Valley fever, blastomycosis, ringworm, and athlete's foot
Protozoa	Malaria, amebiasis, babesiosis, cryptosporidiosis, and giardiasis
Helminths	Intestinal parasites—Ascariasis, trichinellosis, and schistosomiasis
Arthropods	Malaria, encephalitis, Rocky Mountain spotted fever, trypanosomiasis, and leishmaniasis
Source: Data from Friis & Sellers (2009). Epidemiology for public health practice, pp. 440–441.

Infectious agents differ with respect to their ability to infect and cause disease in a host. Other aspects of disease agents are the severity of illnesses they cause and the eventual outcome of infection in the host (Friis & Sellers, 2013). Three terms used in this context are infectivity, pathogenicity, and virulence, which are defined as follows:

Infectivity is the potential of the agent to infect and multiply within a host, causing disease or infection. Polio and measles are diseases of high infectivity.

Pathogenicity refers to the capacity of the agent to cause disease in the infected host. Measles is a disease of high pathogenicity (few subclinical cases). In comparison, polio is a disease of low pathogenicity (most cases of polio are subclinical). The iceberg concept of infection posits that the tip of the iceberg, which corresponds to active clinical disease, accounts for a relatively small proportion of cases and exposures to disease agents. This situation applies particularly well to polio.

Virulence refers to the severity of the disease (i.e., presence of severe clinical manifestations). The rabies virus, which almost always produces fatal disease in humans, is an extremely virulent agent.

The Host

The host refers to the person (or sometimes animal) infected by a disease agent. A primary or definitive host is an organism that harbors the parasite, typically providing nourishment. An intermediate or secondary host harbors the parasite for only a short period of time, during which a developmental stage is completed. For example, in sleeping sickness, the tsetse fly is the primary host and humans are the secondary host.

The human body is equipped with a number of means to reduce the likelihood that an agent will penetrate host defenses, lodge, and cause disease. One group of mechanisms is called nonspecific defense mechanisms against microbial agents. An example of a nonspecific defense is the protection afforded by the skin, which prevents most environmental agents from entering the body. Similarly, the mucosal surfaces also afford protection against foreign invading microbes. Tears and saliva can be thought of as a means to wash away would-be infectious agents. Finally, the high acidity (low pH level) of gastric juices helps to inactivate disease agents that managed to bypass the body's defenses via ingestion of contaminated foods.

Disease-specific defense mechanisms include immunity and the immune system, which also play a central role in warding off invading infectious agents. Immunity relates to the presence of antibodies (proteins produced by the body) that act against disease-causing microorganisms. Immunity can be either active or passive and natural or artificial. We explain these terms in Table 4.2.

Herd immunity refers to the relative immunity a large group or community has against an infectious diseases, based on the large number of members of that population having been immunized (through either past exposure or vaccination). Herd immunity can occur when immune persons prevent the spread of disease to unimmunized individuals. Herd immunity offers protection to the population even though not every single individual has been immunized.

Table 4.2: Types of immunity

Active	A disease organism causes the potential host's immune system to create antibodies against the disease.
Passive	A preformed antibody is administered to a recipient; the immunity is usually of short duration for immune globulin (gamma globulin) derived from the pooled plasmas of adults.
Natural, active	Results from an infection by the agent.
Artificial, active	This type of immunity, also called vaccine-induced immunity, results from an injection with a vaccine that stimulates antibody production in the host. All or part of a microorganism or a modified part of that microorganism is administered to invoke an immunologic response. The response mimics the natural infection but presents little or no risk to the recipient.
Natural, passive	Preformed antibodies during pregnancy are transferred across the placenta to the fetal bloodstream to produce short-term immunity in the newborn.
Artificial, passive	Preformed antibodies against a specific disease are administered to an exposed individual to offer protection against a disease.
Source: Friis & Sellers (2009). Epidemiology for public health practice. Burlington, MA: Jones & Bartlett, pp. 444–445.

After an agent lodges in a susceptible host, a number of events take place in the host before discernible symptoms occur. The incubation period is the time period between a host's exposure to an infection and the first appearance of disease symptoms. During this interval, the infectious organism replicates within the host. The incubation period is often a fixed period of hours, days, or weeks, and provides a clue to the time and circumstance of exposure to the agent.

Environment

The environment, the third component of the triangle, is the unique niche in which disease agents exist, survive, or originate (Friis & Sellers, 2009). Some infectious agents (e.g., fungi) can live in the soil; other agents have animal or human reservoirs.

Certain aspects of the physical environment can promote the survival of disease agents and bring the host and agents into contact. The spread of infectious disease agents in the community is made more likely by poor sanitation, urban crowding, unavailability of clean water, and presence of disease vectors such as rats and mosquitoes (rodents can feed on garbage that has not been disposed of properly; mosquitos can breed in stagnant pools of water).

Poor sanitation and associated living conditions place people at risk for water-, sanitation-, and hygiene-related diseases (CDC, 2012b). Estimates suggest that about one-third of the world's population does not have access to basic sanitation, which includes methods for the safe disposal of human wastes (Friis & Sellers, 2009). Human excreta contain pathogens that cause diarrheal diseases. Contact with human wastes is linked to spread of diseases such as cholera, typhoid, hepatitis, and polio. Unsafe waste disposal involves the use of latrines or open deposition on fields. As the global population continues to grow, the problem of inadequate basic sanitary services is likely to be exacerbated.

One of the most crucial environmental factors in the transmission of infectious diseases is unavailability of clean water. The WHO reported that in 2010 a total of 780 million people in the world did not have access to improved water supplies (CDC, 2012b). An improved water supply is one that has been managed in some way to increase its safety (although drinking water from improved sources is not always safe).

Regional climate patterns are a crucial aspect of infectious disease transmission. Climate, which refers to an area's prevailing weather patterns, directly affects disease transmission by possibly shortening the incubation period as well as shifting the geographic range or movement of the vector or pathogen (Friis & Sellers, 2013). Global geographic areas vary with respect to prevailing climate (e.g., levels of rainfall and mean temperatures). Heavy rainfall in some areas contributes to standing water, which can promote the growth of mosquito populations. Moist, tropical climates promote the survival of diseases such as dengue fever. Another consequence of heavy rainfall is sewage overflow from overtaxed sewage processing facilities. Sewage can then contaminate drinkable water supplies. Rain also can cause ocean contamination by runoff from urban streets. Another climate pattern that could permit the invasion of arthropod vectors into areas where they were formerly uncommon is global warming. In the past, these arthropods were unable to survive during the cold season.

4.4 Mechanisms of Disease Transmission

This section describes modes of disease transmission from the environment to a host. Disease transmission involves an agent's access to a vulnerable location in the body or some means of injection into the body. In order for a new host to become infected, an agent must be transmitted from a reservoir, or the site where infectious agents survive, such as a specific animal or insect. For a human reservoir of disease, the agent needs to be released from a portal of exit. Portals of exit are defined as sites where infectious agents may leave the body; these sites include passages of the respiratory system, the colon (i.e., the pathway in which food enters the body and solid wastes are expelled), skin wounds, and the genitourinary system (the reproductive and urinary system). After the agent has left a portal of exit, it needs to access to a portal of entry, where the agent can enter the body of the new host. The portals of entry for many agents consist of the respiratory system for such diseases as influenza and the common cold; the mouth and digestive system for diseases such as hepatitis A or staphylococcal food poisoning; and the mucous membranes or wounds in the skin for other types of disease (Friis & Sellers, 2013).

Two methods for the transmission of disease agents from a reservoir are direct and indirect transmission.

Direct Transmission

Direct transmission refers to the spread of infectious diseases directly from one person to another through person-to-person contact (Friis & Sellers, 2013). A common form of direct transmission is airborne transmission. Airborne transmission is comprised of airborne droplets that contain infectious particles, which are expelled by the cough or sneeze of an infected host. The particles are then inhaled by another person, and the disease or infection is thus spread. Examples of airborne respiratory tract infections include tuberculosis, influenza, and the common cold.

Photograph of a sneeze in progress.

Centers of Disease Control/Science Faction/SuperStock

A sneeze in progress, revealing the plume of salivary droplets as they are expelled in a large cone-shaped array from this man's open mouth, illustrates a form of direct airborne transmission. The flu and common cold usually spread from person to person when an infected person coughs or sneezes.

Direct transmission of disease agents can occur also during intimate personal contact such as touching, kissing, or sexual intercourse. During contact, skin lesions and the mucous membranes can exude infectious agents from the infected person; in turn, the agents can gain access to an uninfected host. Similarly, sexually transmitted infections (STIs) may be spread when the genitals of one person come into direct contact with the genitals of another person who is infected with an STI. Examples of STIs are syphilis and herpes simplex virus infections. We will describe these conditions in Section 4.5: Infectious Diseases and Community Health.

Even though an infected person may not show active signs and symptoms of an infectious disease (i.e., as a subclinical or asymptomatic case), that individual can transmit the disease to others. An example is a child who has an asymptomatic infection with hepatitis A. Here, even if there are no perceived or observable outward symptoms, the child is ill with this condition and is infectious.

A distinction also needs to be made between acute and chronic infectious diseases. We can recognize an acute infectious disease by the way a person feels, and it is short lived, usually running its course in a matter of days. An example of an acute condition is influenza. A chronic infectious disease lasts weeks or months in duration. An example of this type of infection is HIV/AIDS.

Following an infection, some individuals lapse into a chronic carrier state and can unknowingly transmit infections even though they themselves do not have obvious symptoms. This was the case with "Typhoid Mary" Mallon, a New York City cook who was an alleged asymptomatic carrier of typhoid fever during the early 1900s and who infected over 50 people with typhoid, with several associate deaths from the disease.

Indirect Transmission

Indirect transmission denotes the spread of infection through intermediary sources known as vehicles, fomites, or vectors. Here we present some definitions and examples.

Photograph of a health worker dragging a cloth to capture ticks that carry Lyme disease.

Karen Kasmauski/Science Faction/SuperStock

Indirect transmission can occur through arthropod vectors such as flies, fleas, mosquitos, and ticks. In this photograph, a health worker drags a cloth to capture ticks that carry Lyme disease.

A vehicle for transmission is a contaminated, nonmoving object involved with transmission of disease (Friis, 2009). Examples of possible vehicles are contaminated foods and milk, polluted water, and infectious bodily fluids. For example, when food contaminated with E. coli bacteria are consumed, they can bypass the body's defenses and cause a foodborne illness. Polluted water is associated with waterborne diseases from bacterial, protozoal, and viral infections. Amebiasis, cholera, giardiasis, cryptosporidiosis, and winter vomiting disease are all examples of waterborne diseases. A final example of vehicle transmission is unintentional sticks from "sharps" used in a medical setting. Employees of hospitals may receive unintentional needle sticks from hypodermic needles that are contaminated with infectious blood from patients and develop a blood-borne infection.

A fomite is an inanimate object such as towel, cup, used eating utensils, doorknobs, or medical instruments that are laden with disease-causing agents. In order to prevent the spread of disease from fomites, one should avoid the use of unsanitary utensils and contact with other fomites. In the hospital environment, potential fomites such as bed linen, towels, and used medical supplies need to be processed in a manner to prevent transmission of disease agents from sick patients. Medical personnel should receive instruction in safe procedures for administering injections and disposing of used hypodermic needles.

A vector is a living insect or animal that is involved with transmission of a disease agent. One category of vector includes arthropod vectors and refers to certain species of flies, ticks, fleas, and mosquitos that may carry pathogens. Mosquitos are known to act as vectors for many diseases of great significance for community health; some noteworthy mosquito-borne diseases are West Nile virus, malaria, and viral encephalitis. Ticks are involved with the transmission of Lyme disease and Rocky Mountain spotted fever, fleas can transmit such epidemiologically significant diseases as the plague. Rats and some other rodents can harbor fleas infected with the bacteria that cause plague. For this reason, control of rodent vectors is a crucial dimension of public health.

4.5 Infectious Diseases and Community Health

This section provides a selective review of major infectious diseases that are of great importance for community health. For example, seasonal influenza and foodborne illnesses are dominant causes of infectious disease morbidity in the United States. Table 4.3 provides a list of diseases covered in this section.

Table 4.3: Infectious diseases and related conditions of significance for community and public health
Seasonal influenza	Tuberculosis	Sexually transmitted diseases/infections
Vaccine preventable diseases	Foodborne illnesses	New and emerging infectious diseases
Malaria	Waterborne diseases

Seasonal Influenza

Influenza viruses cause seasonal influenza ("the flu"), an infection of the respiratory system (CDC, 2011a). The flu season transpires during the months of late November through March. Influenza is a disease of great importance due to the severe illnesses and potentially life-threatening complications that it can produce. The CDC (2011, para. 1) states that

in the United States, on average 5% to 20% of the population gets the flu and more than 200,000 people are hospitalized from seasonal flu-related complications. Flu seasons are unpredictable and can be severe. Over a period of 30 years, between 1976 and 2006, estimates of flu-associated deaths in the United States range from a low of about 3,000 to a high of about 49,000 people. Some people, such as older people, young children, pregnant women, and people with certain health conditions are at high risk for serious flu complications.

Immunizations against seasonal flu are an important means for preventing the effects of this condition.

There are three broad types of influenza. Type A infects both humans and animals, with wild birds often acting as hosts. This form of influenza mutates and is responsible for epidemics or pandemics. Type B infects only humans and does not cause pandemics but can cause epidemics. Type C only infects humans, and is less severe than types A and B and does not cause epidemics.

A major pandemic of H1N1 influenza occurred during 2009 and galvanized the public health profession to focus renewed intense attention on the key contributions social distancing measures can make to interrupting transmission of infectious contagious disease. An example of such measures is school closings as part of a comprehensive disease control strategy.

Vaccine Preventable Diseases

Vaccine preventable diseases (VPDs) are infectious/communicable conditions for which preventive immunizations are available. Vaccines are available for several dozen diseases. Immunizations are provided to children and adults, as well as to special exposure groups to prevent the spread and occurrence of these infectious diseases that have been associated with high levels of morbidity and, in some instances, mortality. Table 4.4 provides a list of selected VPDs.

Table 4.4: Examples of vaccine preventable diseases
Cervical cancer (human papilloma virus)	Pertussis (whooping cough)
Diphtheria	Polio
Hepatitis A	Shingles
Hepatitis B	Smallpox
Influenza	Tetanus
Measles	Chickenpox

Hepatitis A and Hepatitis B

Photograph of the San Francisco H1N1 Flu Clinic workers distributing thousands of vaccines.

Getty Images

A man receives an H1N1 flu vaccination during a clinic at the Bill Graham Civic Auditorium on December 22, 2009, in San Francisco, California. The city had 16,000 doses of the vaccine on hand for residents of the city.

Viral hepatitis includes several forms: Hepatitis A and Hepatitis B, which are caused by infection with the hepatitis A virus (HAV) and the hepatitis B virus (HBV) (Friis & Sellers, 2009). Hepatitis A is primarily spread by fecal–oral means (either person-to-person contact or consumption of contaminated food and/or water). Hepatitis B can be spread by adults who engage in risky sexual practices; hence, HBV infection remains more common in high-risk subpopulations than in other groups. HBV is found in lower concentrations in body fluids (e.g., semen and vaginal secretions) than in blood (CDC, 2012c). Consequently, another means for the spread of HBV is contact with blood and blood products through accidental needle sticks.

Measles

Measles, a virus-associated respiratory disease, is highly contagious and is transmitted through the air from infected persons (CDC, 2012d). It is uncommon in the United States and most developed countries as a result of vaccinations. In 1993, the measles vaccine was licensed and a dramatic reduction in the incidence of the condition followed. In 1990, almost 28,000 cases of measles were reported in the United States. Currently, about 61 cases occur annually. Despite the success of the vaccine in reducing the incidence of measles in the United States, measles remains endemic in many parts of the world and has been imported into the country by people who originate from endemic areas.

Photograph of a baby with chickenpox.

Voisin/Phanie/SuperStock

Chickenpox occurred in almost everyone in the United States prior to the varicella-zoster virus vaccine in 1995.

Chickenpox/Shingles

The varicella-zoster virus (VZV) causes both chickenpox and shingles (herpes zoster). Highly contagious, chickenpox produces a blistering rash accompanied by itching and fever. Complications of chickenpox include secondary bacterial infections, pneumonia, encephalitis, and sometimes death. Before the introduction of the varicella vaccine in 1995, almost every person in the United States became infected; children were most likely to be infected. Since the introduction of the vaccine, the incidence of both chickenpox and related hospitalizations has declined markedly. The VZV remains dormant in persons who have had chickenpox and can become reactive later in life causing shingles, a disease associated with a skin rash and, sometimes, a painful condition known as postherpetic neuralgia. Other serious complications (e.g., blindness) from shingles can occur (CDC, 2012e).

Polio

By 1979, polio was eradicated in the United States. In 1988, the World Health Assembly adopted a resolution to eradicate this disease worldwide and organized the Global Polio Eradication Initiative (CDC, 2012f). Since its inception, the total of worldwide polio cases declined from an estimated 350,000 in 1988 to only about 1,300 in 2010. This represents a decline of more than 99% in reported cases (CDC, 2011b). Refer to Figure 4.2 for information on worldwide progress with respect to polio eradication.

Malaria

Malaria, a disease that has plagued mankind for over 4 millennia, derives its name from the Italian words for "bad air." The idea of "bad air" was due to the thought that malaria is associated with poisonous vapors in swamps. The condition is a mosquito-borne disease transmitted by the bite of an Anopheles mosquito and caused by a microscopic parasite known as a Plasmodium (CDC, 2012g). Malaria, which has both uncomplicated and complicated (severe) forms, can produce fatalities—more commonly among infected children in comparison with adults. The CDC (2012h, para 1) reported these facts about malaria worldwide:

A total of 3.3 billion people live in areas at risk of malaria transmission in 106 countries and territories.

The World Health Organization estimates that in 2010, malaria caused 219 million clinical episodes, and 660,000 deaths.

An estimated 91% of deaths in 2010 were in the African region, followed by the South-East Asian region (6%) and the Eastern Mediterranean region (3%). About 86% of deaths globally were in children.

On average, 1,500 cases of malaria are reported every year in the United States, even though malaria has been eliminated from this country since the early 1950s. These cases tend to occur among travelers returning from endemic areas. During the past six decades, 63 outbreaks of locally transmitted mosquito borne malaria have occurred in the United States.

Tuberculosis

Tuberculosis (TB), a disease that normally affects the lungs but can affect other parts of the body, is caused by a bacterium Mycobacterium tuberculosis. The bacteria are transmitted from person to person via airborne dissemination when an individual who has the pulmonary form of TB coughs, speaks, or sneezes (CDC, 2012i).

Globally, TB causes an enormous burden of morbidity and mortality and is endemic in some countries. The WHO states that

[g]eographically, the burden of TB is highest in Asia and Africa. India and China together account for almost 40% of the world's TB cases. About 60% of cases are in the South-East Asia and Western Pacific regions. The African region has 24% of the world's cases, and the highest rates of cases and deaths per capita. (WHO, 2012, p. 2)

Regarding TB trends in the United States, a total of 10,528 cases were reported in 2011 (CDC, 2012j). Since 1993, there has been a steady increase in the incidence of TB cases among foreign-born persons. In 2011, foreign-born persons accounted for 62% of cases nationally. The distribution of TB cases varied according to ethnicity, with the highest incidence among Asians (30% of total cases). The second highest incidence by racial/ethnic group was among Hispanics, who accounted for 29% of TB cases. Figure 4.3 presents the distribution of TB case rates by race/ethnicity in the United States from 2003 through 2011.

Figure 4.3: TB case rates by race/ethnicity, United States, 2003–2011

Source: Adapted from CDC. (2012j, October). Reported tuberculosis in the United States, 2011. Atlanta, GA: U. S. Department of Health and Human Services; CDC.

The rates for tuberculosis among U.S. citizens born in foreign countries have slowly grown over the years, with infection among those of Asian descent being the most common. While almost all of the ethnicities have shown a general trend of decline in tuberculosis cases, there have been several spikes in the Native Hawaiian and Pacific Islander populations.

Source: Adapted from CDC (2012, November 23). Multistate outbreak of Salmonella serotype Bovismorbificans infections associated with hummus and tahini—United States, 2011. Morbidity and Mortality Weekly Report, 61, 945.

The graph indicates those infected with Salmonella by date from August to November of 2011. Studies of incidence rates across the country helped to pinpoint the origin of infection to specific restaurants in metropolitan areas.

Clostridium perfringens—a bacterial agent that sometimes contaminates food such as meats and gravies when they have been inadequately heated or stored at temperatures that permit multiplication of the bacteria. The incubation period is from 10 to 12 hours with symptoms of diarrhea.

Campylobacter—a bacterial agent that can be present in undercooked chicken or pork, raw milk, and other contaminated foods and water. The incubation period is from 2 to 5 days with typical symptoms that include diarrhea, abdominal pain, malaise, and fever.

Staphylococcus aureus—a bacterial agent that develops a toxin when it grows in foods. Technically speaking, staphylococcal food poisoning is an intoxication caused by a toxin and not an infection. The intoxication has a very short incubation period, frequently occurring as soon as 30 minutes after consumption of contaminated food. The primary symptoms are nausea, vomiting, stomach cramping, and diarrhea. Staphylococcus aureus resides on the skin of healthy people (up to 25% of the population) and can be seeded unintentionally into foods by food workers who are harboring the bacteria (CDC, 2006).

Prevention of Foodborne Illnesses

Harmful pathogens and chemicals can enter foodstuffs at any stage during their storage and preparation. Prevention of foodborne illness can take place during any one of these stages and can be accomplished through a number of straightforward procedures:

storing vulnerable foods (e.g., protein-containing foods that can support the growth of microorganisms) at sufficiently low temperatures to prevent the growth of microorganisms

heating food at high enough temperatures to kill bacteria, such as Salmonella, that may be present

avoiding cross-contamination of foods (e.g., contaminating salads with juices from raw poultry)

washing one's hands thoroughly with hot water and soap before preparing foods—a simple, yet powerful means of preventing foodborne disease.

For more information, consult the CDC website for recommendations for keeping families safe from food poisoning at home (CDC, 2011c).

Waterborne Diseases

Photograph of a young woman hiker filtering fresh water from a creek for protection from Giardia.

imagebroker.net/SuperStock

The waterborne disease giardiasis is caused by ingesting the parasite Giardia in unclean drinking water. In this picture, a hiker filters fresh water from a creek for protection from Giardia.

As noted previously, a significant portion of the world's population lacks satisfactory drinking water. Consumption of contaminated drinking water can lead to gastrointestinal illness, reproductive problems, neurological disorders, and other adverse health outcomes. In the United States, quality of water from public supplies is regulated by the Environmental Protection Agency (EPA). Public water supplies in this country are among the safest in the world.

From time to time, waterborne disease outbreaks do occur in the United States. These may result from sewage contamination when processing plants fail or from overflows during heavy rainfall. Contaminants and microbes from agriculture, animal feed lots, and urban streets can find their way into aquifers and groundwater. The leading 10 causes of waterborne illness outbreaks in public water systems are Campylobacter, copper, Cryptosporidium, E. coli O157:H7, Giardia, Hepatitis A, Legionella, norovirus, Salmonella, and Shigella (CDC, 2013d).

Some of these agents were discussed in other sections and are not covered here. However, two parasites that cause waterborne illness are noteworthy. Cryptosporidium is the name of a microscopic parasite associated with a diarrheal disease called cryptosporidiosis, one of the most frequent waterborne diseases in the United States. Cryptosporidiosis can be acquired from drinking water and recreational water (water used primarily for sports such as fishing, swimming, and boating) that are contaminated with the parasite. Another condition that can be spread by unclean drinking and recreational water is giardiasis. This waterborne disease also is caused by a microscopic parasite called Giardia.

Sexually Transmitted Infections

The Mayo Clinic defines sexually transmitted infections (STIs) as "infections generally acquired by sexual contact. The organisms that cause sexually transmitted diseases may pass from person to person in blood, semen, or vaginal and other bodily fluids" (Mayo Clinic, 2013). Examples of sexually transmitted infections (STIs) that challenge community health are HIV infections, gonorrhea, syphilis, chlamydia, and infections with the human papilloma virus (HPV). Bacterial infections such as gonorrhea, syphilis, and chlamydia can be treated with antibiotics, yet they often go undetected or are asymptomatic and go untreated. Possible sequelae (i.e., abnormal conditions that follow and are a result of a disease) of STIs include sterility and harm to fetuses and newborn infants. Progress has been made in the development of programs and interventions for the prevention of STIs. For example, a recent achievement is the availability of a vaccine for immunization against the human papilloma virus, a cause of cervical cancer. Effective interventions, such as encouraging sexually active youth to avoid risky sexual behavior, are available but often not implemented. As a result, STIs remain troubling causes of morbidity and mortality for the community.

As an example of a prevalent and dangerous STI, consider the AIDS epidemic, which has had a major impact upon most American communities. The CDC estimates that

more than 1.1 million people in the United States (US) are living with HIV infection. Nearly one in five (18.1%) of those people are unaware of their infection. Despite increases in the total number of people in the U.S. living with HIV infection in recent years (due to better testing and treatment options), the annual number of new HIV infections has remained relatively stable. However, new infections continue at far too high a level, with approximately 50,000 Americans becoming infected with HIV each year. (CDC, 2012m, p. 1)

Some additional facts regarding HIV:

Men who have sex with men (MSM) accounted for the greatest number of new HIV infections in 2009 and the highest prevalence of HIV infection in 2008.

Black individuals carry the most severe burden of HIV, especially those under 35 years, due to poverty, gay and bisexual relations, and increased rates of untreated STIs.

Black women have the highest incidence of the HIV infection due to poverty, unprotected sex with men, and living with people with HIV.

Approximately 17,774 people died from AIDS in 2009; about 619,400 people died from AIDS since the beginning of the epidemic.

Refer to Figure 4.5 for information on the subpopulations in the United States most affected by HIV.

Figure 4.5: Estimates of new HIV infections in the United States for the most affected subpopulations, 2009

Graph showing the estimated HIV infections by race and sexual orientation in the United States in 2009.

*Intravenous Drug Users

Source: Adapted from Centers for Disease Control and Prevention. (2012, July). HIV in the United States: At a glance, p. 1. Retrieved from http://www.cdc.gov/hiv/resources/factsheets/PDF/HIV_at_a_glance.pdf

There are roughly 50,000 new cases of HIV in the United States each year, with homosexual men accounting for more than half of those infected. Gay men have the highest rate of new infection, regardless of ethnicity. What public health programs, policies, and initiatives could target these at-risk groups?

New and Emerging Infectious Diseases

Other infectious threats arise from microbial agents that are classified as either new or emerging. New and emerging infectious diseases are one of the significant categories of infectious diseases with the potential to impact community health.

'Emerging' infectious diseases can be defined as infections that have newly appeared in a population or have existed but are rapidly increasing in incidence or geographic range. Among recent examples are HIV/AIDS, hantavirus pulmonary syndrome, Lyme disease, and hemolytic uremic syndrome (a foodborne infection caused by certain strains of Escherichia coli)." (Morse, 1995, p. 7)

Among the factors that contribute to emerging diseases are changes in ecological, environmental, or demographic factors that bring people into contact with the infectious agents responsible for these diseases. An example of a factor that was cited previously is deforestation. Another factor is the continuing evolution of microbes and their development of drug resistance. Refer to the Spotlight: Examples of Infectious Disease Threats, Unusual Health Events, and Newly Discovered Pathogens, Worldwide, 2000–2011, for a discussion of these threats.

Spotlight: Examples of Infectious Disease Threats, Unusual Health Events, and Newly Discovered Pathogens, Worldwide, 2000–2011

2000 Outbreak of Rift Valley fever in Saudi Arabia and Yemen, representing the first reported cases of the disease outside the African continent

2001 Anthrax sent in mailed letters, in a terrorist act in the United States

2001 Identification in the Netherlands of a new virus, human metapneumovirus, among children with respiratory infections

2002 First detection of Staphylococcus aureus bacteria completely resistant to vancomycin

2002 Outbreak of multidrug-resistant Salmonella Newport in the United States

2002 Norovirus infection on cruise ships entering U.S. ports

2003 Global outbreak of severe acute respiratory syndrome (SARS) caused by a previously unknown coronavirus

2003 Cases of monkeypox in the United States linked to exotic pets imported from Central Africa

2003 Reemergence of avian influenza A (H5N1) in Southeast Asia, and outbreaks in Africa

2005 Marburg hemorrhagic fever outbreak in Angola

2006 Rift Valley fever outbreak in Kenya

2007 Ebola hemorrhagic fever outbreak in the Democratic Republic of the Congo

2007 First detection in Italy of mosquito-borne transmission of chikungunya fever, previously detected only in parts of Africa and South and Southeast Asia

2007 Discovery in Thailand of a new human species of Bartonella, an insect-borne bacteria that multiples inside red blood cells causing fever, fatigue, muscle pain, headache, and rash

2007 Hemorrhagic fever outbreak in Uganda caused by a new srtain of Ebola: Bundibugyo Ebola virus

2007 Outbreak of Marburg hemorrhagic fever in Uganda

2008 Ebola-like outbreak in Zambia due to a previously unknown virus: Lujo hemorrhagic fever virus

2009–10 Locally transmitted dengue in Florida, representing the first cases acquired in the continental United States outside the Texas–Mexico border since 1945

2009–10 Influenza pandemic caused by a new influenza strain, influenza A (H1N1)

2010 Outbreaks of cholera in Haiti

2011 Outbreak of Shiga toxin-producing Escherichia coli O104:H4 (STEC O104:H4) infections in Germany

2011 Identification by an international team of researchers of a strain of gonorrhea (H041) resistant to all available antibiotics

Source: Data from Centers for Disease Control and Prevention

4.6 Controlling Infectious Diseases

Although infectious diseases are responsible for a major worldwide toll in morbidity and mortality, the situation is not entirely pessimistic, as many procedures and interventions are available for prevention of the spread of infectious diseases. The Centers for Disease Control and Prevention has developed the CDC Framework for Preventing Infectious Diseases. "The ID Framework outlines three critical elements in these efforts: strong public health fundamentals, including infectious disease surveillance, laboratory detection, and epidemiologic investigation; high-impact interventions; and sound health policies" (CDC, 2011d, p. iii). In this section, we will apply the elements stated in the Framework to a discussion of methods for controlling the spread of infectious diseases within the community.

Surveillance Programs and Reportable Diseases

As noted in a Chapter 3, surveillance programs involve the continuous monitoring of infectious diseases. The types of diseases under surveillance are called notifiable and reportable diseases. Ongoing surveillance programs aid in identifying unusual increases in the incidence of infectious diseases and can signal the occurrence of a bioterrorism event caused by the intentional distribution of a hazardous biological agent. Although disease surveillance systems have been in place for many years, interest in developing surveillance systems for bioterrorism increased after 2001 when anthrax spores were distributed intentionally in the U.S. mail and several individuals became ill or died as a result of their exposure.

Outbreak Investigation

Eye Ubiquitous/SuperStock

A woman wears protective clothing while standing amongst the debris left in the aftermath of Hurricane Katrina. The CDC led the effort in investigating an outbreak related to mold exposure following the event in 2005.

Epidemiologic investigations help to identify the sources of infectious disease outbreaks and control them. Investigation of disease outbreaks is the "bread and butter" of local health departments in the United States. Noteworthy outbreaks may first come to the attention of local health departments and then are followed up with investigations by the CDC. Major outbreaks even command the attention of the media. Examples of outbreaks that have occurred include the following:

· Foodborne illness (e.g., salmonellosis and other forms) associated with potlucks, cruise ships, agricultural products, and unsanitary food preparation

· Bacterial illnesses associated with a petting zoo or household pets

· The Legionnaires disease outbreak in Philadelphia in 1976

· Illnesses associated with mold exposure, following major flooding events, such as hurricane Katrina

· Epidemic and pandemic influenza such as the 2009 H1N1 influenza pandemic.

Often disease outbreaks are mysterious, life threatening, and demand immediate epidemiologic investigations in order to bring them under control.

Public Health Laboratories

Public health laboratories encompass both statewide and local public health laboratories that serve specific geographic localities such as a county. Local public health laboratories play a vital function in identifying and controlling infectious diseases. "Local public health laboratories (PHLs) serve an important and evolving role in supporting the broad mission of public health" (Wilson, Gradius, & Zimmerman, 2010, p. 119). Among the goals of public health laboratories are to provide testing of individual patients and aid in investigating local environmental health problems, such as those related to water quality. Also, the laboratories help to support screening and disease control programs. Public health laboratories differ from individual clinical laboratories with respect to their involvement with population-based disease control efforts.

Immunization Programs

According to Healthy People 2020, "Vaccines are among the most cost-effective clinical preventive services and are a core component of any preventive services package" (para 7). Estimates suggest that about 42,000 adults and 300 children die annually in the United States from vaccine preventable diseases. In the United States, unvaccinated individuals and subpopulations are particularly high-risk for vaccine-preventable diseases such as measles that could be introduced by travelers from endemic areas. The impact of morbidity and mortality from vaccine preventable diseases could be reduced substantially by increasing children's and adults' compliance with recommended vaccination schedules.

Public Health Department/Community Health Programs for Communicable Disease Prevention

The range of public health department functions includes the control of sexually transmitted diseases, responding to outbreaks of communicable diseases, maintaining immunization programs, and developing emergency preparedness. Prevention and control of infectious (communicable) diseases is one of the core functions of public health departments. In addition, public health departments are charged with maintaining emergency preparedness in order to respond to disease epidemics and bioterrorism incidents. Depending upon the size and resources of the public health department and the type of community that is served, public health departments offer testing for tuberculosis and sexually transmitted diseases. Also, they engage in case finding and investigation of outbreaks of diseases such as tuberculosis, foodborne illnesses, and waterborne conditions. An important function of health departments is to encourage citizens to become immunized against vaccine preventable diseases.

Policy Initiatives

The establishment of policy initiatives reinforces the control of infectious diseases by delineating societal courses of action, setting priorities, and providing a means for the assessment of policy effectiveness. In the present era of limited governmental resources, policies that are established for control of infectious diseases need to be based on evidence of their need and effectiveness. Data from disease surveillance, laboratory testing, and epidemiologic investigations help to provide the evidence needed to fuel policies and objectives. This section concludes with information from Health Care in Action: Healthy People 2020, a document that lists over 32 objectives under Topic Area of Immunizations and Infectious Diseases.

Health Care in Action: Healthy People 2020 Objectives for Immunization and Infectious Diseases

1. (10 sub-objectives) Reduce, eliminate, or maintain elimination of cases of vaccine-preventable diseases.

2. Reduce early-onset group B streptococcal disease.

3. Reduce meningococcal disease.

4. (4 sub-objectives) Reduce invasive pneumococcal infections.

5. Reduce the number of courses of antibiotics for ear infections for young children.

6. Reduce the number of courses of antibiotics prescribed for the sole diagnosis of the common cold.

Objectives 7–20. (Plus sub-objectives) These objectives relate to maintaining or increasing the immunization levels of various groups, e.g., young children, children in kindergarten, adolescents, and adults.

21. Increase the number of states that use electronic data from rabies animals surveillance to inform public health prevention programs.

22. Increase the number of public health laboratories monitoring influenza virus resistance to antiviral agents.

Objectives 23–28. (Plus sub-objectives) These objectives pertain to reduction of hepatitis A, hepatitis B, and hepatitis C infections.

Objectives 29–32 pertain to tuberculosis (TB), including reducing TB, increasing the treatment completion rate, and encouraging sputum smear-positive persons to complete treatment.

Note: This topic area has a total of 32 objectives.

Source: Healthy People 2020

Summary

Globally and in the United States, infectious and communicable diseases continue to be a major cause of morbidity and mortality. As an example, the category of pneumonia and influenza is the eighth leading cause of mortality in the United States.

The history of civilization has been marked by recurring episodes of devastating epidemics caused by influenza, cholera, and plague. During the 19th century, infectious disease pioneers made great strides in the identification of microbes that caused infectious diseases.

Also, the development of efficacious immunizations for conditions such as smallpox in the 19th century and polio in the 20th century were crucial for the control of vaccine preventable diseases.

The epidemiologic triangle—agent, host, and environment—is helpful in explaining the occurrence of infectious diseases in the community. Agent factors include bacteria and viruses. Host characteristics (e.g., immunity) are related to the lodgment of infectious disease agents. Environmental factors strongly implicated in infectious diseases include quality of general living conditions and access to medical care.

Examples of infectious diseases that are significant for the community are vaccine preventable diseases (e.g., hepatitis, influenza, chickenpox, and polio), sexually transmitted diseases, and foodborne illness. During the last few decades, public health officials have become concerned about the use of infectious agents such as anthrax as instruments of terrorism.

In conclusion, community resources are instrumental in the prevention of infectious/communicable diseases. Protection of the community against infectious diseases is one of the most significant functions of public health departments. They control infectious diseases through immunization programs, outbreak investigations, and promotion of healthful environments.

Study Questions and Exercises

Define the following terms:

infectious disease

inapparent/subclinical infection

active immunity

passive immunity

vaccine preventable disease

herd immunity

Explain the components of the epidemiologic triangle and apply them to the etiology of the following infectious diseases: seasonal influenza, tuberculosis, HIV infections.

Discuss possible host responses to infectious disease agents, explain why all persons who are exposed to microbes do not become ill.

Define and describe direct and indirect modes for the transmission of infectious diseases.

Explain what is meant by the iceberg concept of infection.

Look up the term zoonotic disease. In your opinion, do pet animals (e.g., cats and dogs) present any infectious disease hazards to a household and the community?

What is the significance of foodborne illness to the community? Research one common foodborne illness and suggest methods for its prevention.

In your opinion, how can one prevent the indirect transmission of diseases?

State examples of new and emerging infectious disease agents, bioterrorism agents, and conditions that they cause.

Key Terms

Click on each key term to see the definition.

climate

direct transmission

disease-specific defense mechanisms

epidemiologic triangle

fomite

germ theory of disease

herd immunity

host

incubation period

indirect transmission

infectious agent

infectivity

microorganisms

nonspecific defense mechanisms

pathogenicity

portals of entry

portals of exit

reservoir

sequelae

sexually transmitted infections (STIs)

vaccine preventable diseases (VPD)

vector

vehicle

virulence