Principles of Epidemiology

Diseases that can be transmitted from one host to another, such as measles, colds, and influenza, are communicable diseases. In order for a communicable disease to spread, a specific chain of events must occur. First, the pathogen must have a suitable environment in which to live. That natural habitat, the reservoir, may be on or in an animal, including humans, or in an environment such as soil or water (figure 20.1a). A pathogen must then leave its reservoir in order to be transmitted to the susceptible host. If the reservoir is an animal, the body orifice or surface from which a microbe is shed is called the portal of exit. Disease-causing organisms must then be transmitted to the next host, usually through direct contact or via contaminated food, water, or air (figure 20.1b). They enter the next host through a body surface or orifice called the portal of entry (figure 20.1c).

Diseases that do not spread from one host to another, such as histoplasmosis, are called non-communicable diseases. Microorganisms that cause these diseases most often arise from an individual's normal flora or from an environmental reservoir. Prevention of non-communicable illness is often disease-specific. Histoplasmosis, for example, can be prevented by avoiding undisturbed areas that are likely sources of spores of the causative agent, the fungus Histoplasma capsulatum. This organism thrives in soils that have been enriched with bat, chicken, or bird droppings. ■ histoplasmosis, p. 592

Rate of Disease in a Population

Epidemiologists are most concerned with the rate of a disease, the percentage of a given population infected, rather than absolute number of cases. For example, 100 people in a large city developing disease X in a given period may not be abnor mal, whereas 100 people in a small rural community developing the same disease would indicate a much higher rate and thus be of greater concern to the epidemiologist. A related concept is the attack rate, which is the number of cases developing in a group of people who were exposed to the infectious agent. For example, if 100 people at a party ate chicken that was contaminated with Salmonella, and 10 people came down with symptoms of salmonellosis, then the attack rate was 10%. The attack rate reflects many factors, including the infectious dose of the organism and the immunity of the population.

Other rates are also often used to express the effect of a disease on a population. Morbidity rate is calculated as the number of cases of illness in a given time period divided by the population at risk. Contagious diseases such as influenza often have a high morbidity rate because each infected individual may transmit the infection to several others. Mortality rate reflects the percentage of a population that dies from the disease. Diseases such as plague and Ebola hemorrhagic fever are feared because of their very high mortality rate. The incidence of a disease reflects the number of new cases in a specific time period in a given population at risk, whereas the prevalence reflects the number of total existing cases both old and new in a given population at risk. Usually these rates are expressed as the number of cases per 100,000 people. The prevalence is useful to assess the overall impact of the disease on society because it takes the duration of the disease into account, whereas the incidence rate provides a means of measuring the risk of an individual contracting the disease.

Diseases that are constantly present in a given population are called endemic. For example, both the common cold and influenza are endemic in the United States. An unusually large number of cases in a population constitutes an epidemic. Epidemics may be caused by diseases that are not normally present in a population, such as cholera being reintroduced to the Western Hemisphere, or by diseases that are normally endemic, such as influenza and pneumonia (figure 20.2). A related term is outbreak, which generally implies a cluster of cases occurring during a brief time interval and affecting a specific population; an outbreak may herald the onset of an epidemic. When an epidemic spreads worldwide, such as we have seen with AIDS, it is called a pandemic. Terms commonly used to describe the epidemiology of disease are summarized in table 20.1.

Reservoirs of Infectious Agents

The reservoir of a pathogen is important because it affects the extent and distribution of a disease. For example, coccidioidomycosis is endemic only in the hot, dry, dusty areas of the Western Hemisphere because that is the only habitat of the causative agent, the fungus Coccidioides immitis. Determining the reservoir can help protect a population from disease, because measures can then be instituted to prevent the people from coming into contact with that source. The fact that the United States does not have epidemics of plague, the disease that killed over one-quarter of the European population in the fourteenth

20.1 Principles of Epidemiology 487



Figure 20.1 Spread of Pathogens (a) Reservoirs, (b) transmission, and (c) portals of entry are necessary for the spread of communicable diseases.

Broken skin

Figure 20.1 Spread of Pathogens (a) Reservoirs, (b) transmission, and (c) portals of entry are necessary for the spread of communicable diseases.

488 Chapter 20 Epidemiology

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