Hantavirus pulmonary syndrome is called an emerging disease because of its recent discovery and apparent increase in frequency. Nevertheless, this zoonosis has undoubtedly existed for centuries, occasionally claiming human victims when mice and men get too close together. Since the description of the syndrome, cases have been identified from Canada to Argentina, including several hundred cases from the United States. Most of the cases have occurred west of the Mississippi River and were due to Sin Nombre virus carried by deer mice (figure 23.23). Others have been caused by related viruses carried by other rodents. Outbreaks of the disease correlate with marked increases in mouse populations adjacent to impoverished communities with substandard housing. The virus spreads more easily when the mouse population density is high. Thirty percent or more of the mice can become carriers of the disease. These mice eagerly and easily invade the houses of the poor. Complex ecological factors control the size of mouse populations. The numbers of foxes, owls, snakes, and other predators play a role. Moreover, two outbreaks of hantavirus pulmonary syndrome occurred in association with El Niño weather patterns, which brought increased rainfall to the area, yielding increased plant growth and seed production for mice to feed on. The emergence of han-tavirus pulmonary syndrome is a convincing example of how environmental change can result in infectious human disease. Luckily, despite the large amount of viral antigen in the lung capillaries, few mature infectious virions enter the air passages of the lung; thus, person-to-person transmission occurs rarely, if ever. ■ emerging diseases, p. 498

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