A cluster of eight cases of St. Louis encephalitis was diagnosed among patients in the borough of Queens in New York City in August 1999. The sudden cases of critical brain swelling were found exclusively among the elderly. At about the same time, people noticed an inordinate number of dead crows throughout the city. Other birds, including exotic varieties housed at the Bronx Zoo, were also found dead.
The Center for Disease Control (CDC) was called in to investigate. They found that the West Nile virus, previously found only in Uganda and the Middle East, had been contracted by birds throughout the area, including robins, ducks and eagles. In addition to birds and humans, horses have also been known to be susceptible to the virus, which is spread by mosquitoes.
Upon further investigation, the victims thought to have had St. Louis encephalitis had actually had been infected with West Nile. It causes flu-like symptoms and can be deadly in both the elderly and small children. By the end of the summer, there were 56 confirmed cases of West Nile in New York, though the CDC estimates that 80 percent of people infected with West Nile show no symptoms and therefore would not seek medical help.
In subsequent years, the West Nile virus moved steadily westward across the United States.