The United States is experiencing the worst outbreak of West Nile virus since the mosquito-borne disease was first detected in 1999, health officials said Wednesday.
At least 41 people have died so far this summer and health officials have identified a total of 1,118 cases across the country.
The entire year of 2011 saw just 712 cases and 43 deaths, theCenters for Disease Control and Prevention said.
"The number of West Nile virus disease cases in people has risen dramatically in recent weeks," said Lyle Petersen, the director of the CDC's vector-borne infectious disease division.
Petersen urged the public to protect themselves from mosquito bites and warned that the number of cases is expected to continue to rise.
"The peak of West Nile virus epidemics usually occurs in mid-August, however it takes a couple of weeks for people to get sick, go to a doctor, get diagnosed and then get reported," he said in a conference call.
"Thus we expect many more cases to occur and the risk of West Nile virus infection will probably continue through the end of September."
The outbreak has been concentrated in and around Dallas, Texas, where authorities last week began using aircraft to spray pesticide.
Texas health officials have confirmed 21 deaths and said another four fatalities are being investigated and will likely soon be ruled as caused by West Nile.
The outbreak has also resulted in 323 cases of neuroinvasive disease in Texas alone, nearly all of whom required hospitalization and could face permanent neurological damage.
"Our hearts go out to the families that have been impacted," said Texas health commissioner David Lakey.
It's not yet clear why more people are being sickened this year, but Peterson said the unusually mild winter, early spring and hot summer might have fostered conditions favorable to the spread of West Nile.
The disease, first discovered in Uganda in 1937, is carried by birds and spread to humans by mosquitoes.
The CDC said the uptick could be related to this year's unusually mild winter, but that several factors influence the scope of an outbreak, including the birds that carry the disease, the mosquito population and human behavior.
Health officials said the best way to prevent infection was to wear bug repellent and long sleeves, and to avoid going outside at dawn and dusk, when mosquitoes are most active.
Authorities are also urging Americans to drain standing pools of water where mosquitoes breed.
There are no specific treatments or cures for the disease, though the milder symptoms tend to go away on their own.
About one in 150 people infected will develop severe illness with symptoms that include high fever, convulsions, vision loss, numbness, coma and can cause permanent paralysis and neurological damage.
Eighty percent of those infected will not show any symptoms at all and milder symptoms range from headaches to skin rashes, the CDC said.