Infections caused by the West Nile virus have recently appeared in most parts of the United States. Transmission of the virus to humans occurs through the bite of an infected mosquito. In 2003, doctors confirmed more than 9000 cases of West Nile virus infection in the United States, which caused death in more than 250 patients. Most of the clinical research regarding this disease has focused on patients who have developed infection of the brain (encephalitis) or its coverings (meningitis), conditions that can lead to a type of paralysis known as acute flaccid paralysis. However, only about 20% of infected people have any symptoms that can be attributed to the virus; the rest do not know they have been infected and can be identified only through blood-screening surveys. Most people with symptoms of West Nile virus infection develop fever, loss of appetite, vomiting, eye pain, headache, muscle pain, and skin rash. When these symptoms occur without severe injury to the nervous system, the condition is known as West Nile fever. Very little information is available about the usual clinical course of West Nile fever.