Etiology and Epidemiology

The causative agents of influenza are three myxovi-ruses, the influenza viruses A, B, and C. The B and C viruses are associated with sporadic epidemics among children and young adults, and do not cause pandemics. The A virus is the cause of most cases during and between pandemics. It exists and has existed in a number of subtypes, which usually do not induce cross-immunity to one another. In most instances, influenza viruses pass from person to person by breath-borne droplets, and from animal to animal by this and other routes. Although the disease can spread in warm weather, its epidemics among humans in the temperate zones usually appear in the winters, when people gather together in schools, houses, buses, and so forth under conditions of poor ventilation. Geographically, the malady spreads as fast as its victims travel, which in our time can mean circumnavigation of the globe in a few months, with the pandemic veering to the north and south of the tropics with the changing seasons.

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