Most people who become infected with West Nile virus either have no symptoms or only mild symptoms such as fever, headache, and body aches. Approximately 80% of people with West Nile virus infection will have no symptoms.
About 20% of people with West Nile virus infection will develop "West Nile fever", a mild disease in people, characterized by flu-like symptoms. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.
About one % of those infected with West Nile virus will develop more severe neurologic (brain) disease called "West Nile encephalitis", "West Nile meningitis" or "West Nile meningoencephalitis." Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it. The risk of severe infection is higher among people who are 50 years of age and older.