This virus is spread by the bite of infected mosquitoes and usually causes a mild illness. It may also cause encephalitis (inflammation of the brain), meningitis (inflammation of the lining of the brain a
nd spinal cord) or polio-like paralysis.
This virus was first isolated in the West Nile region of Uganda in 1937, and was first identified in the United States in New York in 1999.
Only a small proportion of mosquitoes are likely to be infected, and less than 1 percent of the people who get bitten by an infected mosquito become seriously ill.
People over age 50 are at greater risk of severe illness.
Mosquitoes are infected when they bite a bird that carries the virus. West Nile virus is not spread from person to person or directly from birds to humans, but some cases have resulted from blood transfusion and organ transplants.
The symptoms of West Nile virus infection may be mild or serious. Mild illness includes fever and muscle aches, swollen lymph glands and sometimes a skin rash. In the elderly, infection may spread to the nervous system or bloodstream and cause sudden fever, intense headache, and stiff neck and confusion, possibly resulting in encephalitis or meningitis. Healthy children and adults may not have any symptoms.
Symptoms appear 3 to 14 days after exposure but may appear as soon as 1 day after exposure or as late as 7 or more days.
A person who gets West Nile virus probably cannot get it again.
There is no specific treatment for the virus. Most people recover from this illness. There is no vaccine for humans.
West Nile virus infection can be prevented by controlling the mosquito population and protecting against mosquito bites.