1993 1994 1995 96 Week and year

Figure 20.2 Incidence of Influenza, an Endemic Disease that Can Be Epidemic Weekly pneumonia and influenza mortality as a percentage of all deaths for 121 cities of the United States, January 1,1992, to February 3,1996.

century, is in part because we recognize the importance of controlling its rodent reservoir. Wild rats, mice, and prairie dogs are the natural reservoir of Yersinia pestis, the bacterium that causes plague. By avoiding a buildup of garbage in cities and homes, we prevent rodent infestation of our living quarters. ■ Coccidioides immitis, p. 591 ■ Yersinia pestis, p. 724

Human Reservoirs

Infected humans are the most significant reservoir of the majority of communicable diseases. In some cases, humans are the only reservoir. In other instances, the pathogenic organism can exist not only in humans but in other animals and, occasionally, the environment as well. If infected humans are the only reservoir, then theoretically the disease is easier to control. This is because it is more feasible to institute prevention and control programs in humans than it is in wild animals. The eradication of smallpox is an excellent example. The combined effects of widespread vaccination programs, which resulted in fewer susceptible people, and the successful isolation of those who did become infected, eliminated the smallpox virus from nature. The virus no longer had a reservoir in which to multiply.

Symptomatic Infections People who have symptomatic illnesses are an obvious source of infectious agents, and ideally they understand the importance of taking precautions to avoid transmitting their illness to others. Staying home and resting while ill both helps the body recover and protects others from exposure to the disease-causing agent. Even conscientious people, however, can unintentionally be a source of infection to others. For example, people who are in the incubation period of mumps still shed virus, as do those who have recovered symp-tomatically from poliomyelitis. ■ mumps, p. 608 ■ incubation period, p. 463 ■ poliomyelitis, p. 677

Table 20.1 Common Terms in Epidemiology



Attack rate

The proportional number of cases developing in the population that was exposed to the infectious agent

Communicable disease

An infectious disease that can be transmitted from one host to another


A disease or other occurrence that is constantly present in a population


A disease or other occurrence whose incidence is higher than expected

Herd immunity

A phenomenon that occurs when a critical concentration of immune hosts prevents the spread of an infectious agent


The number of new cases of a disease in a population at risk during a specified period of time

Index case

The first identified case of a disease in an outbreak or epidemic


Illness. Most often expressed as the rate of illness in a given population at risk


Death. Most often expressed as a rate of death in a given population at risk

Non-communicable disease

A disease that is not transmitted from one host to another


A cluster of cases occurring during a brief time interval and affecting a specific population; an outbreak may herald the

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