WNV affects the central nervous system. However, symptoms may vary. Approximately 80 percent of people (about four out of 5) who are infected with WNV will not show any symptoms.

Up to 20 percent (about one in 5) of the people who become infected will display symptoms which can include fever, headache, body aches, nausea, vomiting and sometimes swollen lymph glands or a skin rash on the chest, stomach and back.

Less than 1 percent (one in 150 people) of individuals infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. West Nile virus infection can be fatal.