West Nile virus is a mosquito-spread infection that usually causes either no symptoms or mild symptoms of headache, fever, body aches, and sometimes a rash and swollen lymph nodes. In rare cases, it can lead to inflammation of the brain (encephalitis) or the tissues surrounding it and the spinal cord (meningitis).

The virus lives in and multiplies in birds. Mosquitoes become infected when they bite and draw blood from infected birds. Mosquitoes may then transmit the virus to humans and animals.

When West Nile virus affects the brain, symptoms may include headache, high fever, stiff neck, reduced attention to surroundings, and disorientation. Symptoms may also include tremors, convulsions, muscle weakness or paralysis, and coma.

No specific treatment is available for West Nile virus infection. Treatment for severe infection involves supportive care in a hospital to help the body fight the illness on its own. In a few cases the infection is fatal.

People usually recover fully from the mild form of West Nile virus infection. But permanent problems may develop in those who have encephalitis, especially children and older people. They may have seizures, memory loss, personality changes, tremors, trouble with walking or balance, or brain damage.