osquitoes can transmit several viruses that cause inflammatory brain diseases in humans. The most common of these infections in the United States, for example, is St.Louis encephalitis (SLE), and epidemic outbreaks are strongly associated with periods of a few days when temperatures exceed 30° C. Particularly wet late winter months, followed by summer drought may exacerbate the threat. Global warming in the United States may result in a more frequent and more northerly occurrence-even up to Canada-of a disease that is currently limited mainly to southern parts of the country. SLE passes through several bird species, including sparrows, pigeons and blackbirds. Its epidemic vector is often mosquitoes that commonly breed in sewage or waste-water ponds. For this reason, SLE tends to break-out in urban or suburban areas. The first major epidemic was in St.Louis in 1933 (the driest year since 1837). Since 1980, there have been outbreaks in Florida, Mississippi, New Orleans, Texas, Arizona, California and Colorado.