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




Propagated source epidemic


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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