West Nile Virus

West Nile Virus (WNV) is:

  • An infection that can cause serious disease. WNV is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall.
  • It is spread by the bite of an infected mosquito. Mosquitoes become infected by feeding on infected birds. In a very small number of cases, WNV has also been spread directly from an infected person through blood transfusions, organ transplants, breastfeeding and during pregnancy from mother to baby.

Signs & Symptoms


No symptoms in most people. Most people (70-80%) who become infected with West Nile virus do not develop any symptoms.

Febrile illness in some people. About 1 in 5 people who are infected will 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 symptoms in a few people. Less than 1% of people who are infected will develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues).
  • The symptoms of neurologic illness can include headache, high fever, neck stiffness, disorientation, coma, tremors, seizures, or paralysis.
  • People with certain medical conditions, such as cancer, diabetes, hypertension and kidney disease are also at greater risk for serious illness.
  • Recovery from severe disease may take several weeks or months. Some of the neurologic effects may be permanent.
  • About 10% of people who develop neurologic infection due to West Nile virus will die.