West Nile Virus causes an infection that is spread to humans when bitten by certain types of mosquitoes. Mosquitoes become infected by biting infected birds. The purpose of this study is to find out more about West Nile Virus, which may assist in the design of better treatments. The study will be looking at the effects of the disease on individuals, specifically their nervous systems, and the outcomes of the disease. Study participants will include 120 adults ages 18 and older, who have either a fever and/or changes in their neurological (involving the brain and nervous system) status due to documented West Nile Virus infection. Study procedures will include physical examinations, blood tests, urine tests, cerebral spinal fluid (fluid in and around the brain and spinal cord) tests, and neurological (nervous system) testing. Study participants will be followed for 12 months.