A mosquito-transmitted virus causes most cases of West Nile infection. Most people infected with West Nile virus either don't develop signs or symptoms or have only minor ones, such as a fever and mild headache. However, some people develop a life-threatening illness that includes inflammation of the spinal cord or brain.

Mild signs and symptoms of a West Nile virus infection generally go away on their own. But severe signs and symptoms — such as a severe headache, fever, disorientation or sudden weakness — need immediate attention.

Exposure to mosquitoes where West Nile virus exists increases your risk of getting infected. You can lower your risk by protecting yourself from mosquitoes by using mosquito repellent and wearing clothing that covers your skin.


Most people infected with the West Nile virus have no signs or symptoms.

Mild infection signs and symptoms

About 20% of people develop a mild infection called West Nile fever. Common signs and symptoms include:

  • Fever
  • Headache
  • Body aches
  • Vomiting
  • Diarrhea
  • Fatigue
  • Skin rash

Serious infection signs and symptoms

In less than 1% of infected people, the virus causes a serious nervous system (neurological) infection. This may include inflammation of the brain (encephalitis) or of the membranes surrounding the brain and spinal cord (meningitis).

Signs and symptoms of neurological infections include:

  • High fever
  • Severe headache
  • Stiff neck
  • Disorientation or confusion
  • Coma
  • Tremors or muscle jerking
  • Seizures
  • Partial paralysis or muscle weakness
  • Vision loss
  • Numbness

Signs and symptoms of West Nile fever usually last a few days. But signs and symptoms of encephalitis or meningitis can linger for weeks or months. Certain neurological effects, such as muscle weakness, can be permanent.

When to see a doctor

Mild symptoms of West Nile fever usually resolve on their own. Seek medical attention right away if you have signs or symptoms of serious infection, such as severe headaches, a stiff neck, disorientation or confusion. A serious infection generally needs hospitalization.


West Nile virus generally spreads to humans and animals from bites of infected mosquitoes. Mosquitoes get infected and carry the virus after biting infected birds. You can't get infected from casual contact with an infected person or animal.

Most West Nile virus infections happen during warm weather, when mosquitoes are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — generally ranges from four to 10 days.

West Nile virus has appeared in Africa, Asia, Europe and the Middle East. It appeared in the United States in the summer of 1999, and since then has been reported in every state — except Hawaii and Alaska — as well as in Canada.

Other possible routes of transmission

In a few cases, West Nile virus might have spread through other routes, including organ transplants and blood transfusions. However, blood donors are screened for the virus, greatly reducing the risk of infection from blood transfusions.

There also have been reports of some transmission of the virus from mother to child during pregnancy or breastfeeding or exposure to the virus in a lab, but these are rare.

Risk factors

Most cases of West Nile virus in the United States occur June through September. Cases have been reported in all 48 lower states.

Risk of serious infection

Even if you're infected, your risk of developing a serious West Nile virus-related illness is very small. Less than 1% of people who are infected become severely ill. And most people who do become sick recover fully. You're more likely to develop a severe or fatal infection based on:

  • Age. Being older puts you at higher risk.
  • Certain medical conditions. Certain diseases, such as cancer, diabetes, high blood pressure (hypertension) and kidney disease, increase your risk. So does receiving an organ transplant.


Your best bet for preventing West Nile virus and other mosquito-borne illnesses is to avoid exposure to mosquitoes and remove standing water, where mosquitoes breed.

  • Unclog roof gutters.
  • Empty unused swimming pools or empty standing water on pool covers.
  • Change water in birdbaths and pet bowls regularly.
  • Remove old tires or unused containers that might hold water and serve as a breeding place for mosquitoes.
  • Install or repair screens on windows and doors.

To reduce your exposure to mosquitoes:

  • Avoid unnecessary outdoor activity when mosquitoes are most common, such as at dawn, dusk and early evening.
  • Wear long-sleeved shirts and long pants when outdoors.
  • Apply mosquito repellent containing an Environmental Protection Agency-registered insect repellent to your skin and clothing. Choose the concentration based on the hours of protection you need. The higher the percentage (concentration) of the active ingredient, the longer the repellent will work. Follow the directions on the package, paying special attention to recommendations for use on children.
  • When outside, cover your infant's stroller or playpen with mosquito netting.

Dec. 19, 2020
  1. West Nile virus. Centers for Disease Control and Prevention. https://www.cdc.gov/westnile/index.html. Accessed Nov. 30, 2020.
  2. Petersen LR. Epidemiology and pathogenesis of West Nile virus infection. https://www.uptodate.com/contents/search. Accessed Nov. 30, 2020.
  3. Petersen LR. Clinical manifestations and diagnosis of West Nile virus infection. https://www.uptodate.com/contents/search. Accessed Nov. 30, 2020.
  4. AskMayoExpert. West Nile virus infection. Mayo Clinic; 2019.
  5. Petersen LR. Treatment and prevention of West Nile virus infection. https://www.uptodate.com/contents/search. Accessed Nov. 30, 2020.
  6. Tosh PK (expert opinion). Mayo Clinic. Dec. 1, 2020.