Epidemic Investigation

Epidemic Curve

Exposure is a common source but for a brief period of time, like a common meal consumed by all people attending say a marriage.
Number of cases rise rapidly to a peak and falls off gradually
Majority of cases occur within one incubation period

Exposure is continuous over a prolonged period of time like days, weeks or longer as in a contaminated well in a village
Cases are spread over a longer period of time with gradual increase in cases, which plateau off and then gradually decline
Cases spread over more than one incubation period

Spreads from one susceptible host to another either directly (person-to-person) or via an intermediate host. e.g. Covid-19
Series of of irregular peaks reflecting the number of generations of infection
Multiple peaks separated by approximately one incubation period

Exposure is intermittent e.g., contaminated food packets sold over period of time
Multiple peaks but no relation to the incubation period (reflects intermittent times of exposure)

Cases that stand apart from the overall pattern such as the first, or index case (early onset) or late onset cases are called as outliers.
Outliers provide information
Early onset cases could be unrelated case, a source of epidemic, or a person exposed much earlier
Late onset cases could be unrelated case, a long incubation period, or a person exposed later than others

An epidemic curve

Shows magnitude of epidemic.
Shows distribution of cases over time.
Helps to distinguish epidemic from endemic disease.
Shows where we are in time course of epidemic - beginning, declining or end and what the future course could be like.
Probable time period of exposure in a disease with known incubation period can be identified.
Provides clues about the spread of the disease (point source, common continuous source, propagated epidemic or intermittent source).

References