After a disease outbreak, epidemiologists race to define characteristics such as the person, the place, and the time. These data are then used to compile a list of possible risk factors involved in the spread of disease. The significance of these hypothesized risk factors can be tested in an analytical study.
Figure 20.4 Incidence of Tetanus by Age Group
Determining the profile of those who become ill is critical to defining the population at risk. Variables such as age, sex, race/ethnicity, occupation, personal habits, previous illnesses, socioeconomic class, and marital status may all yield clues about risk factors for developing the disease. For example, the fact that adults are more likely than children to develop tetanus may indicate that they are not receiving adequate immunizations, which are recommended once every 10 years (figure 20.4).
The geographic location of disease acquisition identifies the general site of contact between the person and the infectious agent. This helps pinpoint the exact source. The location may also give clues about potential reservoirs, vectors, or geographical boundaries that may affect disease transmission. For example, malaria can only be transmitted in regions that have the appropriate mosquito vector.
The timing of the outbreak may also yield helpful clues. A rapid rise in the numbers of people who became ill suggests that they were all exposed to a single common source of the infectious agent, such as contaminated chicken at a picnic. This type of outbreak is called a common-source epidemic. If the numbers of ill people rise gradually, the disease is likely contagious, with one person transmitting it to several others, who each then transmit it to several more, and so on. This type of pattern is
Figure 20.5 Comparison of Propagated Versus Common Source Epidemics The graph depicts the number of new cases that develop over a period of days.
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