Illustration of the seasonal variation of a vector borne disease like dengue. An example of an ‘endemic channel’ is shown here. The ’expected increase in cases’ usually coincides with, or follows, the rainy season. The shaded area corresponds to an ‘alarm zone’ where case numbers reach levels above the mean (or median) of a preceding time period (for example 5 years). The ‘epidemic zone’ is entered when case numbers reach levels above 2 standard deviations (or the third quartile).