What is the West Nile virus?

A virus is a tiny life form that can grow inside a person or animal and may cause illness. West Nile virus is a type of virus that is widespread in Africa, the Middle East, and parts of Asia. It is now found in the United States, emerging in the summer of 2002.

How do you get West Nile virus?

The virus spreads by mosquitoes that become infected when they bite birds that are carrying the virus. When an infected mosquito then bites a person or animal, the virus is injected through the mosquito's saliva into the bite wound.

In addition, four novel (non-mosquito) modes of transmission (spreading) have been documented: through the placenta from mother to baby, through breast-feeding; through the transplantation of infectious organs; and through transfusion of infectious blood products.

How common is West Nile virus?

Many people get mosquito bites, but few get ill from West Nile virus. Most people infected with the virus have very mild or no symptoms. Less than 1 percent of people infected with West Nile virus get seriously ill.

What are the symptoms of West Nile virus?

In most cases, West Nile virus causes no illness or a mild self limiting febrile (feverish) illness. In some cases, the virus can cause a mild illness called West Nile fever. The symptoms of West Nile fever resemble flu symptoms and may include:

  • Fever
  • Headache
  • Body aches
  • Skin rash
  • Swollen glands

Less frequently, West Nile virus can cause a more serious illness, such as West Nile meningo-encephalitis – which is inflammation (swelling) of the brain – or meningitis – which is inflammation of the protective membrane surrounding the brain and spinal cord. Symptoms of serious illness may include:

  • Headaches
  • High fever
  • Stiff neck
  • Decreased mental abilities
  • Reduced alertness
  • Involuntary movement of muscles (tremors or convulsions)
  • Muscle weakness
  • Paralysis
  • Coma

In rare cases, West Nile virus can cause death.

Last reviewed by a Cleveland Clinic medical professional on 01/16/2017.


  • Centers for Disease Control and Prevention. West Nile virus Accessed 1/23/2017.
  • National Institute of Allergy and Infectious Diseases. West Nile Virus Accessed 1/23/2017.

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