In August 1999, an outbreak of encephalitis caused by West Nile virus (WNV) was detected in New York City and subsequently identified in neighboring counties and reported to the U.S. Centers for Disease Control and Prevention (CDC). In response, an extensive mosquito-control and risk-reduction campaign was initiated, including aerial and ground applications of mosquito adulticides throughout the affected areas.

No human WNV infections were found in New York City with an onset date after the campaign was completed.