West Nile virus is a viral infection characterized by symptoms similar to influenza. It effects numerous species, including humans. The disease is transmitted by mosquitoes, which are the primary vector for human infection. It is found in both temperate and tropical zones.

Most people infected with West Nile virus will suffer no symptoms at all. However, about 20% will develop West Nile fever, which will usually resolve itself within about one week. However, some of those patients will develop life-threatening encephalitis. Only about 1% of persons exposed to the virus will die from the infection, but among persons who develop the encephalitis, the mortality rate is closer to 5%,

There is no effective treatment for the illness, so treatment for it is generally supportive and based on the symptoms being suffered by the patient. Anti-retrovirals are only considered if encephalitis develops. There is no completely effective vaccine for the disease, so prevention efforts concentrate on mosquito control, which effectively prevents the disease from being widespread. At its peak in 2002, there were only about 4,000 reported cases in the United States.

Individuals who contract the encephalitis often have a lengthy period of recovery.

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