Epidemic Theory

Epidemics can occur unexpectedly, as when a new disease enters a community, or can occur regularly at certain times of the year, as in epidemics of measles. Epidemic contrasts with endemic, which means the continuous presence of an infection in the community and is described by incidence and prevalence measurements. This section will cover epidemics and how they are measured.

Epidemic means an excess of cases in the community from that normally expected, or the appearance of a new infection. The point at which an endemic disease becomes epidemic depends on the usual presence of the disease and its rate. With an unusual disease, a few cases could be an epidemic, whereas with a common disease (e.g. gastroenteritis), an epidemic occurs when the usual rate of the disease is substantially exceeded. Criteria can be set so that when the number of cases exceeds this level the epidemic threshold is crossed. The epidemic threshold can either be the upper limit of cases expected at that particular time, an excess mortality, or a combination of both the number of cases and the mortality.

Characteristics of an epidemic (Fig. 2.1) are as follows:

1. Latent period, the time interval from initial infection until start of infectiousness.

2. Incubation period, the time interval from initial infection until the onset of clinical disease. The incubation period varies from disease to disease and for a particular disease has a range. This range extends from a minimum incubation period to a maximum incubation period (see Chapter 19).

3. Period of communicability, the period during which an individual is infectious. The infectious period can start before the disease process commences (e.g. hepatitis) or after (e.g. sleeping sickness). In some diseases, such as diphtheria and streptococcal infections, infectiousness starts from the date of first exposure.

Various factors modify the incubation period so that if it is plotted on a time-based graph, it is found to rise rapidly to a peak and then tail off over a longer period (Fig. 2.2). The infecting dose, the portal of


Incubation period


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  • Mohan P. Gupta
    Some more characteristics of epidemic can be added to above list:<br />- unstable host-infectious agent relationship as opposed to stable host-infectious agent relationship in endemic<br />- Infections would usually be exogenous, means the infections most of the time have entered the community from outside.<br />-there will be both temporal (time) and spatial (place) clustering of cases.<br />- the balance among the Agent, Host and Environment (THE TRIAD) must have been tipped in favour of agent. (Reminds me of &quot;The Tipping Point&quot; by Malcolm Gladwell.<br /><br />By the way where are the Fig 2.1 and Chapter 19
    8 years ago

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