(CNN) -- The recent West Nile virus outbreak is the largest ever seen in the United States, according to new numbers from the Centers for Disease Control and Prevention.

The number of cases so far this year is the highest recorded through August since the disease was first detected in the United States in 1999. As of August 21, 38 states had reported human infections. The cases reported to the CDC total 1,118, including 41 deaths.

"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 Dr. Lyle Petersen, the director of the CDC's Vector-Borne Infectious Disease Division. "Thus we expect many more cases to occur."

Petersen said that the reason for the high number of cases this year is unclear, but that unusually warm weather could have fostered favorable conditions for the disease's transfer to humans.

About 75% of the cases 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.

Here are five things you need to know about West Nile virus:



1. Most mosquitoes do not carry West Nile.

In areas where mosquitoes carry the virus, only about one in 500 mosquitoes is infected, according to the Connecticut Mosquito Management Program.

2. Most people bit by West Nile mosquitoes do not get sick.

About 80% of people bit by a mosquito infected with the West Nile virus do not get sick, according to the CDC. About 20% will have relatively mild symptoms, such as fever, headache and vomiting. Symptoms can last for as short as a few days or as long as a few weeks. About one in 150 people infected with West Nile will develop a severe illness, which can include paralysis, coma or death.

3. You can help prevent West Nile with the "four Ds."

• Use mosquito repellent with DEET

• Dress in long pants and long sleeves

• Be especially careful at dusk and dawn

• Drain any standing water, such as kiddie pools or bird fountains, where mosquitoes like to breed.

4. People over 50 are most vulnerable.

Those older than 50 are the most likely to become severely ill with West Nile and should take special care to avoid mosquitoes, according to the CDC.

5. Seek medical care immediately if you have severe headaches or confusion.

If you develop symptoms of severe West Nile virus illness, such as unusually severe headaches or confusion, seek medical attention immediately, according to the CDC. Severe illness usually requires hospitalization. Milder cases improve on their own and do not necessarily require medical attention.