others. For example, in the western United States, the geographical distribution of St. Louis encephalitis virus might expand north into areas in which transmission typically is blocked by cool temperatures (Hess et al., 1963). At the same time, the range of western equine encephalitis virus might shift from the southern part of the country northward, to cooler, more temperate habitats.

Other possible effects of global warming, such as increased precipitation or a rise in sea level, could affect the distribution of vectors that rely on water to complete their life cycles. For example, some mosquito species might alter their range and thus come into contact with new viruses or hosts; other species might become extinct in certain areas.

A change in climate can also affect the survival of infections agents, particularly viruses, outside their hosts. Humidity can favor or hinder the transmission of pathogenic agents. The seasonality of some human viral diseases (such as influenza A) may be due to climatic factors that exert an influence on the organism, its host, or both; sudden alterations in the climate could dramatically affect disease incidence.

The ability of infectious agents to adapt to changed conditions is considerable. Whether global warming occurring over an extended period of time would have any appreciable effects on these organisms, their distribution, or their ability to cause disease is unknown. Based on current knowledge, however, this committee believes that the impact of population growth (particularly when it leads to high population density) is likely to have a more predictable effect on the emergence of infectious diseases than the projected changes in global temperatures.

INTERNATIONAL TRAVEL AND COMMERCE

Travel

Travel, which involves the movement of people and microbes from one region to another, has always contributed to the emergence of infectious diseases. Whether new diseases emerge depends on the novelty of the microbe being introduced, its transmissibility, and the existence of an environment suitable for maintaining the disease and its agent. It is important to distinguish between transient introductions or acquisitions of novel diseases, which are common, and the establishment and propagation of a new pathogen, which are rare. Two examples of such establishment are syphilis and smallpox. According to the view that is still most widely held, syphilis is believed to have been introduced into Europe by sailors returning from the New World (Ampel, 1991). European explorers are believed to have introduced smallpox to the Americas (Crosby, 1972).

Until its global eradication in 1977, smallpox was frequently spread by travel. In 1940, nine years before the disease was eradicated in the United



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