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How Common Is West Nile Encephalitis?

In areas where the virus is circulating, very few mosquitoes are infected with the West Nile virus. Even if the mosquito is infected, not everyone who gets bitten by an infected mosquito actually develops West Nile virus symptoms.
Even if symptoms do develop, they are usually mild. In fact, less than 1 percent of people who get bitten and become infected will develop West Nile virus encephalitis. The chances of you developing West Nile encephalitis from a single mosquito bite are extremely small.

Risk Factors

There are certain factors that increase a person's chance of developing West Nile encephalitis. Specific risk factors include:
  • Living in areas where virus activity has been identified.
  • Being over 50 years of age.

Preventing West Nile Encephalitis

The easiest and best way to avoid West Nile encephalitis is to prevent mosquito bites.
(Click West Nile Virus Prevention for strategies that can help prevent a West Nile virus infection.)


People typically develop symptoms between 3 and 14 days after they are bitten by an infected mosquito.
When a person becomes infected, his or her possible symptoms can vary. About 1 in 150 people infected with West Nile virus will develop West Nile encephalitis symptoms. Symptoms of West Nile encephalitis can include:
  • Flu-like symptoms
  • Stupor
  • Disorientation
  • Coma
  • Tremors
  • Convulsions
  • Muscle weakness
  • Vision loss
  • Numbness
  • Paralysis.
Written by/reviewed by:
Last reviewed by: Arthur Schoenstadt, MD
Last updated/reviewed:
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