WASHINGTON - US health authorities on Wednesday said the recent West Nile virus outbreak is the largest ever experienced in the country, and warned it could get worse.

According to new numbers announced by the Centers for Disease Control and Prevention (CDC) in a telephone news briefing, 38 US states had reported a combined 1,118 human infections of the lethal virus as of Tuesday, of which, 41 people have died. That toll rose to 42 on Wednesday when a new death was reported in the state of Arkansas.

The number of infection cases this year is the biggest recorded through August since the disease was first detected in the United States in 1999.

Dr. Lyle Petersen, director of the CDC's Vector-Borne Infectious Disease Division, said the reason for the high number of infection cases this year is unclear, but unusually warm weather could have fostered favorable conditions for the mosquito- borne virus' spreading to humans.

Because it takes some time for symptoms to develop and cases to be reported, Petersen said CDC predicted that infection cases would keep increasing through the end of September.

"The peak of West Nile virus epidemics usually occurs in mid- August, but it takes a couple of weeks for people to get sick, go to the doctor and get reported," said Petersen. "Thus we expect many more cases to occur."

About 75 percent of the cases reported so far this year are in Texas, Mississippi, Louisiana, South Dakota and Oklahoma. Texas has been at the epicenter of the outbreak, with 586 confirmed cases and 21 deaths, according to the Texas Department of State Health Services.

The West Nile virus is a mosquito-borne illness that is prevalent in temperate and tropical regions and can lead to serious neurological disease in some cases, and its symptoms include sudden onset of fever, headache, nausea, dizziness and muscle weakness.

Scientists say about 80 percent of human infections show no symptoms.