T t t t t t t t t t t t

Time i i Latent period ^

Period of communicability

Fig. 2.1. Parameters of an infection (see text for definitions).

Fig. 2.2. Distribution curve of incubation times (the epidemic curve).

entry, immune response of the host and a number of other factors modify the normal distribution to extend the tail of the graph. By using a log-time scale, this skewed curve can be converted to a normal distribution and the mean incubation period measured.

An epidemic can either be a common source epidemic or propagated source epidemic (Fig. 2.3).

Common source epidemics can further be divided into a point source epidemic resulting from a single exposure, such as a food poisoning episode, or an extended epidemic resulting from repeated multiple exposures over a period of time (e.g. a contaminated well). In a propagated source epidemic, the agent is spread through serial transfer

Common source epidemics

Point source

Extended source

Time

Time

Time

Propagated source epidemic

Time

Fig. 2.3. Epidemic types.

from host to host. With a disease having a reasonably long incubation period, the initial peaks will be separated by the median incubation periods. Chickenpox (varicella) can start as an epidemic in one school; then mingling children will lead to transfer to another school, leading to a series of propagated epidemics.

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