West Nile virus (WNV) is an infectious disease that first appeared in the United States in 1999. Infected mosquitoes spread the virus that causes it. People who get WNV usually have no symptoms or mild symptoms. The symptoms include a fever, headache, body aches, skin rash, and swollen lymph glands. Febrile illness (fever) in some people. About 1 in 5 people who are infected develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash. Most people with this type of West Nile virus disease recover completely, but fatigue and weakness can last for weeks or months. Severe illness can occur in people of any age; however, people over 60 years of age are at greater risk. People with certain medical conditions, such k, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk.Recovery from severe illness might take several weeks or months. Some effects to the central nervous system might be permanent.