West Nile virus is a mosquito-borne virus that can infect humans, birds, horses and other mammals. In most humans, West Nile virus infection causes a mild or moderate, short-lived flu-like illness, or causes no symptoms at all. However in some cases, particularly among persons 50 years of age and older, it can cause serious neurological diseases such as encephalitis, meningitis, or acute flaccid paralysis. West Nile virus first appeared in North America in New York City in 1999. Since then, the virus has spread across the continental United States. Visit www.cdc.gov for more information on West Nile virus nationally.

Most people who are infected with West Nile virus either have no symptoms or experience a mild or moderate illness with symptoms such as fever, headache, fatigue, or body aches before fully recovering. Some persons may also develop a rash or swollen lymph glands. In some individuals, particularly persons 50 years of age and older, West Nile virus can cause serious disease that affects the brain and spinal tissue. Severe illness may include encephalitis (inflammation of the brain), meningitis (inflammation of the membrane around the brain and spinal cord), or acute flaccid paralysis (a polio-like syndrome in which muscles become very weak or paralyzed). Symptoms of more severe disease may include headache, high fever, stiff neck, nausea and vomiting, confusion, coma, tremors, convulsions, muscle weakness and/or paralysis. At its most serious, West Nile virus can cause permanent neurological damage and death. Among those people who need to be hospitalized for West Nile virus, 10-15% die of their illness. People who do develop symptoms normally become ill 3-15 days following the bite of an infected mosquito.