## Investigation of a common source epidemic

In the investigation of any outbreak of a disease, the basic approach is to gather information on the following:

1. Persons: age, sex, occupation, ethnic group, etc. comparing the number infected with the population at risk.

2. Place: country, district, town, village, household and relationship to geographical features such as roads, rivers, forests, etc. conveniently marked on a map.

3. Time: annual, monthly (seasonal), daily and hourly (nocturnal/diurnal). The number of cases occurring within each time-period is plotted on a graph. These aspects will be covered in greater detail later.

In a point source epidemic, the number of cases of the disease occurring each day are plotted on a graph to produce an epidemic curve. The earliest cases will be those with the minimum incubation period and the last of the cases are those with the maximum incubation period if all were infected at a single point in time, as illustrated in Fig. 2.4.

Three factors describe a point source epidemic:

• the incubation period of the disease;

If only two of these factors are known, then the third can be deduced. From the epidemic curve, the median (or geometric mean) of the incubation periods is determined. If the disease is known from its clinical features, then the incubation period will also be known (Chapter 19). Therefore, by measuring this known incubation period back in time from the median incubation period on the curve or the minimum incubation period from the beginning of the curve, the time of infection can be calculated. The source now localized to a restricted period of time can be more easily investigated.

If the disease is unknown, but there is evidence of the time of infection (e.g.

 Median or geometric j mean incubation period / 1 \ / 1 \ / 1 \ / 1 / 1 / 1 / 1 / 1 / 1 v Curve of specific disease x Time onset of disease Minimum \ Incubation Maximum . 1 times

Investigation of a point source epidemic.

Investigation of a point source epidemic.

In a new infection, everyone will be at risk (e.g. with the SARS virus), but as the infection spreads, persons will become immune and are therefore no longer at risk. Where an epidemic occurs at regular intervals (e.g. measles), only those people who have not met the infection before or have not been vaccinated will be at risk.

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