West Nile Virus

West Nile is a virus that spreads through mosquito bites. Most people infected with West Nile virus don’t have symptoms. About 1 in 5 people have symptoms like fever, rash and muscle aches. Rarely, West Nile can cause serious brain and spinal cord inflammation (encephalitis and meningitis).


Symptoms of West Nile virus include fever, headache, muscle aches, nausea, vomiting, diarrhea, rash and more.
Most of the time, symptoms of West Nile virus are flu-like. Less than 1% of people develop severe nervous system symptoms.

What is West Nile virus?

West Nile is a virus that spreads through mosquito bites. Most people who get infected don’t have symptoms. But about 1 in 5 people have a fever, headache, body aches and other flu-like symptoms (sometimes called West Nile fever). Rarely, West Nile virus infects your nervous system and causes serious brain or spinal cord inflammation (encephalitis or meningitis).

West Nile is named after the West Nile district of Uganda, where it was first identified.

How common is West Nile virus?

West Nile virus is found in many parts of the world, including North America, Europe, Africa, the Middle East, Australia and Asia. It’s the most common mosquito-transmitted virus in the U.S., with cases reported in 49 states. There have been over 51,000 symptomatic cases in the U.S. since the first cases in the country in 1999.


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Symptoms and Causes

What are the symptoms of West Nile virus?

Symptoms of West Nile fever include:

West Nile encephalitis and meningitis

Symptoms of more serious West Nile infections include:

  • Intense, very painful headache.
  • High fever (above 103 degrees Fahrenheit or 39.5 degrees Celsius).
  • Stiff neck. You might not be able to move your chin toward your chest.
  • Confusion.
  • Muscle weakness.
  • Muscle movements you can’t control (tremors or convulsions).
  • Seizures.
  • Paralysis.
  • Coma.

What causes West Nile virus?

West Nile is an arbovirus, or a virus you get from an arthropod (arthropods are a large group that includes insects). It’s an RNA virus in the genus Flavivirus. Similar viruses cause dengue fever, yellow fever and Zika.


How do you get West Nile virus?

Infected mosquitos transmit West Nile virus. They usually get the virus by biting an infected bird (there’s no evidence humans get it directly from birds). The virus multiplies inside the mosquito, and it’s transmitted to you (or another animal) when it bites you. The incubation period — how long before you have symptoms — is usually two to six days after getting bitten (but can be up to 14 days).

In a very few cases, West Nile virus has spread:

  • From a pregnant person to their fetus.
  • Through human milk.
  • Through a blood transfusion.
  • Through an organ transplant.

How contagious is West Nile virus?

West Nile isn’t contagious. You can’t get it from another person who has it.

What are the risk factors for West Nile fever?

Mosquitos can bite anyone. But certain people are at a higher risk of getting severely ill if they get West Nile. You might be at a higher risk if you:

What are the complications of West Nile virus?

Less than 1% (about 1 in 150) of people infected with West Nile will get severe symptoms. The most common complication is swelling in part of your nervous system (your brain and spinal cord), including encephalitis and meningitis.

Long-term effects of a severe West Nile infection could include:


Diagnosis and Tests

How is West Nile virus diagnosed?

A provider can test your blood or cerebrospinal fluid (CSF) to look for antibodies or other signs of a West Nile virus infection. They usually only do these tests if you’re seriously ill. If you have signs of brain inflammation, they’ll get images of your brain with a CT scan (computed tomography scan) or MRI (magnetic resonance imaging).

Management and Treatment

How is West Nile virus treated?

There aren’t any antiviral medications that treat West Nile virus. You can treat mild symptoms at home with over-the-counter (OTC) medications like you’d take for a cold or flu.

If you have serious neurological symptoms, providers will monitor you at a hospital. They might treat your symptoms or try to reduce brain swelling with:


Can you prevent West Nile virus?

There isn’t a vaccine to prevent West Nile virus. The best way to reduce your risk is to protect yourself from mosquito bites. Ways to do this include:

  • Avoiding high-risk times. Stay inside when mosquitos are most active, especially early in the morning and around sunset.
  • Using insect repellent. Spray insect repellent on exposed skin or clothes before going outdoors. Check for ones that are Environmental Protection Agency (EPA)-registered for effectiveness.
  • Covering your skin. Wear lightweight clothes like long pants or a long-sleeved shirt to cover more of your body when you’re outdoors.
  • Draining standing water. Regularly empty and clean any areas that may collect water, such as a birdbath or clogged rain gutters, where mosquitos could breed.
  • Keeping mosquitos outside. Keep doors and windows screened or closed to keep mosquitos from entering your home.
  • Taking precautions when traveling or sleeping outdoors. Sleep with the windows closed where possible, or use a mosquito net to keep from being bitten at night.

Outlook / Prognosis

Can you ever get rid of West Nile virus?

Yes, in most people, their immune systems eventually fight off West Nile virus. Flu-like symptoms usually go away on their own. But if you have a weakened immune system, it may be difficult to get rid of the viral infection. Nervous system damage from severe West Nile infections can be permanent, even after the virus goes away.

How long does West Nile virus last?

Mild symptoms of West Nile usually fully resolve in a few weeks. Sometimes, fatigue and muscle weakness can linger for months. After a nervous system infection, you may have long-lasting or permanent memory loss, hearing loss, balance or gait issues, and other neurological conditions.

Is West Nile virus fatal?

West Nile is fatal in about 1 out of 10 people with nervous system infections (or about 1 in 1,500 people with West Nile).

Living With

When should I see my healthcare provider?

If you think you could have West Nile virus, talk to a healthcare provider. They can let you know:

  • Whether you need to be tested.
  • What symptoms of serious illness to look out for.
  • How to take care of yourself at home.
  • When to follow up with them.

When should I go to the ER?

Go to the emergency room if you have symptoms of severe illness, including:

  • Severe headache.
  • High fever.
  • Stiff neck.
  • Confusion.
  • Muscle weakness or tremors.
  • Seizures.

What questions should I ask my doctor?

It might be helpful to ask your healthcare provider:

  • When can I expect to feel better?
  • How do I manage my symptoms at home?
  • What serious symptoms should I look out for?
  • When should I follow up with you?

A note from Cleveland Clinic

Most of the time, mosquitos are just an obnoxious nuisance. But they sometimes carry viruses that can make you sick. Fortunately, 80% of people who have West Nile never develop symptoms. And most people’s risk of getting seriously ill is very low.

If you might be around mosquitos, you can take steps to protect yourself from their bites. If you do get bitten, see a healthcare provider if you develop symptoms of West Nile. They can help you understand what serious symptoms to look out for and answer any questions you have.

Medically Reviewed

Last reviewed on 10/16/2023.

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