Tracey McNamara had never seen anything like it. In July, dead crows began turning up by the dozens at New York City's Bronx Zoo. By August, McNamara, the zoo's staff pathologist, had collected carcasses of 40 birds. Meanwhile Dr. Deborah Asnis, an infectious-disease specialist at Flushing Hospital in Queens, reported the admission of two elderly patients with muscle weakness, fever and confusion.
Although unsuspected at the time, these two seemingly unconnected events presaged an alarming encephalitis epidemic. By last week lab tests revealed that the disease had stricken 36 people in metropolitan New York and caused at least five deaths in...