Topic 2: The Epidemic Curve
The epidemic curve represents in a graphic form the onset of cases of the disease, either as a histogram, a bar graph, or a frequency polygon. The frequency of new cases (or outbreaks) is plotted on the ordinate (y-axis) over a time scale on the abscissa (x-axis). A typical epidemic curve may have four segments:
1. the endemic level,
2. an ascending part,
3. a plateau,
4. a descending part, and, at times a secondary peak.
The first limb of the curve which represents the endemic level, i.e. the expected level of disease, should be drawn first. The actual epidemic curve is then superimposed. An epidemic is said to prevail when the frequency of cases (or outbreaks) in a population clearly exceeds the normally expected level for a given area and season,
Pandemic - an epidemic takes international proportions.
A secondary peak in an epidemic is usually due to:
1. introduction of susceptible animals into the previously epidemic area, or
2. movement of infected animals from the epidemic area and contact with susceptible animals.
The main peak of the curve is at times preceded by a smaller peak which could represent the index cases, i.e. the first cases to occur. The interval between this first peak and the beginning of the next or main peak could indicate the incubation period.