During the late summer of 1999, New York City recorded an unusual number of cases of encephalitis (inflammation of the brain). At the same time, the Bronx Zoo reported a massive death of birds and mammals.
The human encephalitis cases might have been attributed to a flare-up of an endemic arbovirus (a virus transmitted by a tick or mosquito bite) such as St. Louis encephalitis, but the concurrent bird and mammal deaths suggested the human illnesses warranted further investigation.
Scientists eventually identified these as the first confirmed cases of West Nile virus (WNV) in North America.
