West Nile virus is transmitted by mosquitoes, who get it by biting infected birds, according to the Centers for Disease Control and Prevention.
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About four out of five people who become infected by a mosquito bite have no symptoms.
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However, about 20% of people who become infected with West Nile virus develop West Nile fever, within two to 15 days afterward.
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Symptoms include fever, headache, tiredness, body aches, occasionally with a skin rash (on the trunk of the body) and swollen lymph glands. While the illness can be as short as a few days, even healthy people have reported being sick for several weeks.
There is no treatment for West Nile virus fever, the CDC says.
About one in 150 infected with West Nile virus will develop a severe form of the disease called neuroinvasive disease, according to the CDC. This includes West Nile encephalitis, West Nile meningitis and West Nile poliomyelitis.
Symptoms of these diseases include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.
Anyone who develops a high fever with severe headache should consult their health care provider.
People over age 50 and the immunocompromised (for example, transplant patients) are at the highest risk for the severe form of the disease.
One in 1,000 people who get West Nile virus die.