ATLANTA -- Health officials say West Nile virus season is off to a menacing start, with more serious illnesses reported so far this year than any since 2004.

Through the end of July, 241 human cases have been reported in 22 states, including four deaths. The bulk are in Texas, especially around the Dallas area.

The Centers for Disease Control and prevention released the numbers Wednesday. Health officials believe the mild winter, early spring and very hot summer have fostered breeding of the mosquitoes that spread the virus to people.

CDC officials noted that most West Nile infections are reported in August and September, so it's not clear how bad this year will be.