The outbreak of the West Nile virus this year has been "one of the largest" since the virus first appeared in the country in 1999.
Cases are up 40 percent since last week and may rival the record years of 2002 and 2003, federal health officials said Wednesday.
So far this year, 1,590 cases of the mosquito-borne disease have been reported to the Centers for Disease Control and Prevention, and 66 deaths.
About half of the cases are serious illnesses, and the CDC considers those the best indicator of West Nile activity because many mild cases do not get reported and their symptoms may not even be recognized.
"We're in the midst of one of the largest West Nile outbreaks ever seen in the United States," said Dr. Lyle Petersen, a CDC official.
Typical symptoms are fever, headache and body aches, and most people get better on their own in a few days. Less than 1 percent develops neurological symptoms such as stiff necks and even coma and paralysis.
Based on reports of West Nile so far this year, "we think the numbers may come close" to those of 2002 and 2003, when nearly 3,000 severe illnesses and more than 260 deaths occurred each year, said the CDC's top expert on the disease, Dr. Lyle Petersen.
Health officials think that West Nile activity will peak in mid-to-late August, but likely will continue through October. Because symptoms can take two weeks to appear, reporting cases lags behind when people became infected.
The disease first appeared in the United States in 1999. Officials say this year's early spring and hot summer may have contributed to the current boom in cases. Mosquitoes get the virus from feeding on infected birds and then spread the virus to people they bite.
All states except Alaska and Hawaii have found West Nile virus in people, birds or mosquitoes this year. Texas has been the hardest hit, accounting for half of the cases reported to the CDC so far.
"I'm not convinced that we have peaked. We may have plateaued," said Dr. David Lakey, commissioner of the Texas Department of State Health Services.
The CDC also says it does not expect Hurricane Isaac to have much of an impact on cases in Southern states. Heavy storms can wash out mosquito breeding grounds, although standing water can aid breeding, Petersen said. Many other factors, such as the population of infected birds, influence the severity of West Nile outbreaks, he said.
Based on reporting by the Associated Press.