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The epidemiology of influenza is an excellent model for the study of virus spread within a population. While symptoms can be severe, in part due to host factors, the virus infection is localized, and the virus is efficiently cleared from the host. Flu viruses have evolved unique mechanisms to ensure constant generation of genetic variants, and the constant appearance of new influenza virus serotypes leads to periodic epidemics of the disease. Some of these mechanisms are described in detail in Chapter 15, Part IV. The respiratory distress caused by most strains of flu virus is not particularly life‐threatening for healthy individuals, but poses a serious problem for older people and individuals with immune system or respiratory deficiencies. Some strains of the virus cause more severe symptoms with accompanying complications than others. At least one strain, the Spanish strain of 1918–1919 (H1N1), caused a worldwide epidemic with extremely high mortality rates in the years immediately following World War I.