In August 1999, Dr. Deborah Asnis, an infectious-disease clinician in Queens, New York, reported two cases of encephalitis associated with muscle weakness to the New York City Department of Health. Dr. Marci Layton and her colleagues at the Department of Health rapidly mounted a collaborative investigation that ultimately identified West Nile virus as the cause of the ongoing outbreaks of disease in humans and in crows. West Nile virus, which was first identified in a patient in Uganda in 1937, had not previously been recognized in the Western Hemisphere. The virus has now spread throughout most of the United States . . .