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Old 11-07-2014, 10:12 AM   #1
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Default Principles of Epidemiology

A 500 page or so self study course on disease surveillance or investigation. The course was developed by the Centers for Disease Control and Prevention.


As a sample, from page 56:

Epidemic patterns
We sometimes classify epidemics by how they spread through a population, as shown below:
• Common source
— Point
— Intermittent
— Continuous
• Propagated
• Mixed
• Other

A common source outbreak is one in which a group of persons is exposed to a common noxious influence, such as an infectious agent or a toxin. If the group is exposed over a relatively brief period, so that everyone who becomes ill develops disease at the end of one incubation period, then the common source outbreak is further classified as a point source outbreak. The epidemic of leukemia cases in Hiroshima following the atomic bomb blast and the epidemic of hepatitis A among college football players who unknowingly drank contaminated water after practice one day each had a point source of exposure (11, 21). When the number of cases in a point source epidemic is plotted over time, the resulting epidemic curve classically has a steep upslope and a more gradual downslope (a so-called “log-normal distribution”). Figure 1.20 is an example of the typical log-normal distribution of a point source outbreak.

In some common source outbreaks, cases may be exposed over a period of days, weeks, or longer, with the exposure being either intermittent or continuous. Figure 1.21 is an epidemic curve of a common source outbreak with continuous exposure. When we plot the cases of a continuous common source outbreak over time, the range of exposures and range of incubation periods tend to dampen and widen the peaks of the epidemic curve. Similarly, when we plot an intermittent common source outbreak we often find an irregular pattern that reflects the intermittent nature of the exposure.

An outbreak that does not have a common source, but instead spreads gradually from person to person—usually growing as it spreads—is called a propagated outbreak. Usually transmission is by direct person-to-personcontact, as with syphilis. Transmission may also be vehicleborne, as the transmission of hepatitis B or HIV by sharing needles, or vectorborne, as the transmission of yellow fever by mosquitoes.

In a propagated epidemic, cases occur over more than one incubation period. In theory, the epidemic curve of a propagated epidemic would have a successive series of peaks reflecting increasing numbers of cases in each generation. The epidemic usually wanes after a few generations, either because the number of susceptibles falls below some critical level, or because intervention measures become effective.
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