Other flaviviruses carried and spread by mosquitoes include those that cause yellow fever, Japanese encephalitis, St. Louis encephalitis, and dengue. The West Nile virus was first discovered in the U.S. in 1999. It was originally found in Uganda in 1937. The virus is found most often in the U.S., Africa, West Asia, Europe, and the Middle East. In the U.S., the virus has developed into a summertime illness that continues into the fall. This season is generally June through September.

An infected bird is bitten by a mosquito. The mosquito picks up the virus and passes it on to another bird. People can become infected when a female mosquito carrying the virus bites a person. Crows, jays, and magpies seem most vulnerable to the virus, but other birds can become infected. More than 40 species of mosquito can pass on the virus. The virus can't be spread through casual contact such as touching or kissing an infected person. Experts have found that the virus can be passed from person to person through blood transfusions or organ transplants. It also can be passed from birth parent to child through breastmilk.

Severe illness can happen at any age, but people older than 60 are at greater risk. This is especially true for people with a weakened immune system. Certain jobs put a person at higher risk of getting bitten by an infected mosquito. These include farmers, foresters, landscapers, gardeners, painters, roofers, pavers, construction workers, laborers, mechanics, and other outdoor workers.

Up to 1 in 5 people infected with the virus will have symptoms. Symptoms include fever, headache, body aches, nausea, vomiting, swollen lymph nodes, and a skin rash. Symptoms often develop 3 to 14 days after a bite by an infected mosquito. Four out of 5 people infected with the virus will not have any symptoms at all.

About 1 person in 150 will develop severe illness after getting infected by the West Nile virus. This is called West Nile encephalitis or West Nile meningitis, depending on which parts of the central nervous system are affected. A person with either of these illnesses often must be treated in a hospital. In some cases, these illnesses can be fatal. Symptoms of these severe illnesses include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. These symptoms may last several weeks or longer.

No medicines currently are available to treat the infection caused by the virus. No vaccines are available to prevent infection. Treatment may include hospitalization, IV (intravenous) fluids, and breathing support as needed. The only way to prevent infection is to not get bitten by a mosquito. Testing is available that can help diagnose the infection in people who have symptoms of viral encephalitis or meningitis.

Wearing long sleeves, long pants, and socks can help prevent mosquito bites. This is especially true when the clothing is sprayed with an insect repellant such as permethrin. Use products that contain DEET on your skin. Don't stay outdoors at dawn and dusk. These are the times of day when mosquitoes are more likely to bite. If you open your windows during the summer, make sure they have screens. Drain any standing water around your home. This includes water buckets, pool covers, and flowerpots.