It's still too early to say how bad the year will end up because most infections are reported in August and September. But never before have so many illnesses been reported this early, said Dr. Lyle Petersen, of the Centers for Disease Control and Prevention.
"We're in the midst of one of the largest West Nile outbreaks ever seen in the United States," said Petersen, who oversees the CDC's mosquito-borne illness programs.
So far, 1,118 illnesses have been reported, about half of them in Texas. In an average year, fewer than 300 cases are reported by mid-August. There have also been 41 deaths this year, the CDC said.
And cases seem to be accelerating: about 400 of the cases were reported in just the last week.
Experts think the mild winter, early spring and very hot summer helped stimulate mosquito breeding and the spread of the virus. Mosquitoes pick up the virus from birds they bite and then pass it on to people.
CDC officials are also looking into the possibility that the virus mutated, but so far have no information showing that happened, Petersen said.
West Nile virus was first diagnosed in Uganda in 1937, but no cases were reported in the U.S. until 1999 in New York. The virus gradually spread across the country.
It peaked in 2002 and 2003, when severe illnesses reached nearly 3,000 and deaths surpassed 260. Last year was mild, with fewer than 700 cases.
Only about 1 in 5 infected people get sick. Early symptoms can include fever, headache and body aches. Some recover in a matter of days. But 1 in 150 infected people will develop severe symptoms including neck stiffness, disorientation, coma and paralysis.
Many illnesses probably go unreported, especially milder cases. In this year's case count, more than half are severe, CDC officials said.
In recent years, cases have been scattered across the country. Hot spots are usually in southeast Louisiana, central and southern California, and areas around Dallas, Houston, Chicago and Phoenix.
Those areas seem to have a combination of factors that include the right kinds of virus-carrying mosquitoes and birds, along with large numbers of people who can be infected, health officials say.
Illnesses this year have been reported in 38 states, but the bulk of them have been in Texas, with a concentration in the Dallas area.
It's not clear why Texas is seeing so many cases, but it's alarming. Twenty-one deaths have been reported in the state so far this year, which is more than all other years combined. Four of the Texas deaths were reported on Tuesday.
Officials last week started aerial spraying for mosquitoes in Dallas County. But it's too soon to measure the effect - it takes between three and 14 days for people to develop symptoms after being infected by a mosquito.
The best way to prevent West Nile disease is to avoid mosquito bites. Insect repellents, screens on doors and windows, and wearing long sleeves and pants are some of the recommended strategies. Also, empty standing water from buckets, kiddie pools and other places to discourage breeding.