The West Nile virus season is on track to be the mildest in seven years, with less than a third the number of serious cases as last year's total, U.S. health officials said.
As of Tuesday, there were 368 severe cases, with 18 deaths, according to preliminary reports. Mississippi and California were hardest hit, together accounting for nearly half the cases.
Most West Nile infections are reported in August and September, so health officials believe the worst of the season is probably over.
It's not clear why this season has been so mild, said officials at the Centers for Disease Control and Prevention. It's the fewest cases since 2001, when the mosquito-borne virus was still emerging in the United States and was only reported in 10 states.
Mosquitoes often pick up the virus from birds they bite and then spread it to people. Perhaps the weather in some areas of the country was not as favorable for mosquito-breeding as in years past, some experts said.
West Nile virus was first reported in the United States in 1999 in New York, then gradually spread across the country.
About one in five infected people get sick. One in 150 will develop severe symptoms including neck stiffness, disorientation, coma and paralysis.
For all of 2007, more than 1,200 cases of severe West Nile illness were reported, and 124 deaths. The peaks occurred in 2002 and 2003, when severe illnesses numbered nearly 3,000 and deaths surpassed 260.