Fig. 1.

The seasonal and latitudinal distribution of outbreaks of type A influenza in the world, 1964–1975, summarized from the Weekly Epidemiological Record of the World Health Organization into major zones. The diagrams show for each calendar month the percentage of each zone's total outbreaks. In both north and south temperate zones the epidemics are distributed around the local midwinter, whereas the tropical zones show a transition, each approximating towards the distribution of its own temperate zone. The curve indicates the ‘midsummer’ path taken annually by vertical solar radiation. The ‘epidemic path’ seems to parallel it, but to lag 6 months behind it. (Reproduced with permission, Cambridge University Press, Hope-Simpson, 1981.)