Map shows states where West Nile virus has been detected in humans and mosquitos. The disease can cause a range of symptoms, from fever to brain swelling.

• West Nile virus is the most common mosquito-borne disease in the US.
• The virus is spread by mosquitoes during the summer and fall.

At least 18 states have detected West Nile virus in mosquitoes or humans as of mid-July, according to the Centers for Disease Control and Prevention. Based on CDC data from the past 20 years, the peak of the season is yet to come.

West Nile virus (WNV) is commonly spread from infected mosquitoes to humans throughout summer and fall, but only about one in five people infected with the virus experience symptoms of disease, the CDC says. Most of the cases reported to the CDC involve severe illness, as milder symptoms such as fever can be confused with other viruses.

Many states test local mosquito populations for WNV beginning in late spring, with some health departments reporting the virus as early as May.

Residents are warned to use insect repellent, wear long sleeves and pants outside, and dump standing water where mosquitoes might breed. People who are older or immunocompromised should take extra care to avoid mosquitoes, as they have a higher risk of getting severely ill, according to the Illinois Department of Public Health.

Local health departments are meant to report all human WNV disease cases to the CDC, but some states and counties report their mosquito surveillance data independently.

The most recent CDC data, as of June 27, shows a total of 17 human cases of WNV disease across the US this year. Several states have reported WNV in mosquitoes without any known infections in humans, as people in those areas should take precautions to avoid mosquito bites.

Human infections reported to the CDC are usually serious

Most people who are infected with WNV do not experience symptoms, but the disease can be fatal if it reaches the brain. About 1 in 150 people infected experience severe neurological symptoms, including headache, stiff neck, convulsions, vision loss, paralysis, and coma, according to the CDC.

Of the 15 WNV disease cases reported to CDC last year, 11 patients were classified as having neuroinvasive disease, which involves swelling in the brain or spinal cord.

Only one person died of WNV disease last year: an individual in Arizona, according to the most recent CDC data. Another death was reported in Oklahoma but has not yet been included in the CDC's official count.

There is no specific treatment or vaccine available for WNV, so the only way to prevent serious illness is to avoid mosquito bites.

States test mosquitoes and other animals to get ahead of the virus

Local and state health departments regularly trap and test mosquitoes to monitor them for diseases, including WNV. At least 12 states including Florida, Massachusetts, and New York have detected WNV in mosquitoes but not in humans. States with known human cases, such as Texas and Arizona, may also have insect and animal infections.

Mosquitoes become infected with WNV when they bite infected birds. They can also pass the virus back to healthy birds through their bites or if the birds eat them, continuing the cycle of infection.

Some states monitor bird populations to keep tabs on WNV, rather than trapping and testing mosquitoes. Delaware, for instance, has detected the virus not in humans or mosquitoes, but in chickens at a state monitoring station. The state health department warned people to be aware of the disease in livestock as well, as horses can become infected with the virus.