Influenza epidemics account for 250,000 to 500,000 deaths globally each year. These epidemics are markedly seasonal, occurring in temperate climates in the northern and southern hemispheres during their respective winters. By contrast, in the tropics, there is significant influenza activity year round, with a peak in the rainy season (Figure Q23–3). Can you suggest some possible explanations for the patterns of influenza epidemics in temperate zones and the tropics?

Figure Q23–3

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northern southern hemisphere hemisphere tropics JFMAMJJASONDJFMAM JJASON D month cases of influenza