West Nile Virus Activity — United States, 2007

West Nile virus (WNV) is the leading cause of arboviral encephalitis in the United States. Originally identified in Africa in 1937, WNV was first detected in the western hemisphere in 1999 in New York City. Since then, WNV has caused seasonal epidemics of febrile illness and neurologic disease in the United States. This report summarizes national WNV surveillance data for 2007. WNV transmission to humans or animals expanded into 19 counties that had not reported transmission previously and recurred in 1,148 counties where transmission had been reported in previous years. A total of 1,227 cases of WNV neuroinvasive disease (WNND) and 117 deaths were reported. These findings highlight the need for ongoing surveillance, mosquito control, promotion of personal protection from mosquito bites, and research into additional prevention strategies, including a WNV human vaccine.