A: The United States had the biggest outbreak of West Nile encephalitis in the world in 2002. There were 2,500 cases of encephalitis and meningitis caused by a virus carried by mosquitos and 232 deaths. A total of 3,800 cases were reported, but about one-third of those were West Nile fever, a much less serious disease. (Figures are from the U.S. Centers for Disease Control and Prevention, Atlanta.)

A: Possible reasons are it is a new virus in this country, so most animals and people had never been exposed to it. Also, the U.S. has a large elderly population, one of the most susceptible groups.

A: Many states have increased their mosquito-control programs and made other efforts to stop the disease's spread.

A: Nearly all parts have reported cases of West Nile virus. West Nile virus showed up first in New York in 1999. As of the end of 2002, only four Western states in the lower 48 appeared free from the virus: Oregon, Nevada, Utah, and Arizona.