Health ConditionsInfectious Diseases

What Is West Nile Virus?

What Is West Nile Virus?
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Published on April 29, 2025
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West Nile virus is primarily transmitted to humans via mosquito bites; it also affects birds and mammals like horses.

 The virus is the leading cause of mosquito-transmitted disease in the United States,

 and around 80 percent of people infected with the disease do not experience any symptoms.

In about 20 percent of infected people, West Nile virus evolves into a febrile illness (sometimes called West Nile fever), with symptoms including fever, headache, body aches, vomiting, diarrhea, and rash. In rarer cases (about 1 in 150 infected people), West Nile virus can lead to severe and potentially fatal neurological diseases like encephalitis or meningitis. Elderly and immunocompromised people are most at risk for severe disease.

There is no vaccine or medicine to treat West Nile virus in humans. The best way to prevent infection is to protect yourself against mosquito bites. Treatment for symptomatic West Nile virus typically involves hospital care.

Types of West Nile Virus

The 1 in 5 people who develop symptoms from West Nile virus experience one of two types: febrile illness and, less commonly, severe neurological illness that can take multiple forms.

Signs and Symptoms of West Nile Virus

The symptoms of West Nile virus depend on the form the disease takes when it occurs. Those with febrile illness may experience the following symptoms in addition to fever:

  • Headache
  • Body aches
  • Joint pain
  • Vomiting
  • Diarrhea
  • Rash

Additionally, people with febrile illness may experience fatigue or weakness for an extended period — up to weeks or even months — following recovery.

How West Nile Virus Affects the Body shows Fever, Headache, Joint Pain, Vomiting, Body Aches, Diarrhea, Rash. Everyday Health logo.
Most people infected by West Nile virus experience no symptoms, but 20 percent of people infected experience some or all of these symptoms.Everyday Health

Those who develop neurological disease, which takes the form of brain inflammation (encephalitis) and inflammation of the membranes surrounding the brain and spinal cord (meningitis), among others — may experience the following symptoms:

  • High fever
  • Headache
  • Stiffness in the neck
  • Stupor
  • Disorientation
  • Convulsions
  • Tremors
  • Muscle weakness
  • Loss of vision
  • Numbness
  • Paralysis
  • Coma

Causes and Risk Factors of West Nile Virus

In the vast majority of cases, West Nile virus is caused by bites from mosquitos that picked up the virus by feeding on infected birds.

 In very rare instances, the virus is spread through exposure in a lab, blood transfusions, organ transplants, or from mother to infant via the placenta or breastfeeding. It is not spread through coughing, sneezing, or touching infected people or animals, and you can’t get it from eating infected animals.

Anyone can be bitten by a mosquito and infected, but some people have factors that increase their risk of severe illness. These include:

How Is West Nile Virus Diagnosed?

A healthcare provider may diagnose West Nile virus on the basis of a person’s signs, symptoms, and history of possible exposure to infected mosquitos.

 The provider will also perform a blood test and in some cases a lumbar puncture, sometimes called a spinal tap.

Treatment and Medication Options for West Nile Virus

There are no vaccines, medicines, or other specific treatments for West Nile virus in humans. In generally healthy people with mild febrile illness, rest, fluids, and over-the-counter medications may be enough to relieve symptoms until the infection runs its course.

Treatment in more severe cases typically involves a hospital stay and supportive therapies, such as:

  • Intravenous fluids
  • Breathing support such as a ventilator
  • Monitoring and prevention of secondary infections, such as pneumonia
  • Nursing care

Prevention of West Nile Virus

There are no medical options to prevent infection, but you can protect yourself by taking precautions against mosquito bites. Recommendations include:

  • Insect repellent
  • Long-sleeved, loose-fitting clothing
  • Treat clothing and outdoor gear with permethrin, an insecticide that kills or repels mosquitos
  • Screens and other devices for preventing exposure to mosquitoes in indoor and outdoor spaces

How Long Does West Nile Virus Last?

Febrile illness from West Nile virus typically lasts a few weeks, though weakness and fatigue can continue for weeks or months afterward. Symptoms such as paralysis, motor impairments, hearing loss, and other neurological conditions may last permanently in more serious cases. West Nile virus is fatal in about 1 in 1,500 people who experience neurological symptoms.

Complications of West Nile Virus

People with febrile illness may experience fatigue or weakness for weeks or months after initial onset. Possible long-term effects of a neurological infection include:

  • Memory loss
  • Hearing loss
  • Gait disorders or difficulty walking
  • Weakness
  • Abnormal reflexes
  • Depression

Research and Statistics: How Many People Get West Nile Virus?

There were 1,466 total human cases of West Nile virus illness in the United States in 2024.

 In the past 10 years, the number of cases in the country has varied from as low as 731 in 2020 to as high as 2,911 in 2021. The states with the highest number of reported cases between 1999 and 2023 are California, Colorado, Texas, Nebraska, and Arizona.

The Takeaway

  • Four out of five people infected with West Nile virus never experience any symptoms.
  • The best way to prevent infection is to protect yourself against mosquito bites.
  • Most cases of the virus are febrile, with flu-like symptoms.
  • Neurological illness from the West Nile virus is rare but may cause permanent side effects or be fatal in extremely rare cases.

Common Questions & Answers

How do you get West Nile virus?
West Nile virus is mostly transmitted through bites from infected mosquitos. There are other ways it can be transmitted — such as through lab exposure, a blood transfusion, an organ transplant, or from mother to infant through the placenta or breastfeeding — but these cases are very rare.
People infected with West Nile virus do not experience any symptoms in 4 out of 5 cases. One in five people experience a febrile, flu-like illness sometimes known as West Nile fever. In very rare instances — fewer than 1 percent — West Nile virus causes a severe neurological illness like meningitis or encephalitis.
A healthcare professional will diagnose you based on your symptoms and likelihood of exposure to infection. There are also blood tests and cerebrospinal fluid tests that can detect the presence of the virus.
Cases of febrile illness may be treated like a typical flu-like illness with rest, fluids, and over-the-counter medicines. More serious cases may require hospital care. There are no specific medicines or vaccines for West Nile virus in humans.
Most people with the virus make a full recovery. Those with febrile illness may experience weakness or fatigue for weeks or months afterward. People with neurological illness from the virus may experience permanent neurological effects, including memory or hearing loss, difficulty walking, and depression.

Resources We Trust

EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Resources
  1. Transmission of West Nile Virus. Centers for Disease Control and Prevention. May 15, 2024.
  2. About West Nile. Centers for Disease Control and Prevention. May 15, 2024.
  3. West Nile: Symptoms, Diagnosis, & Treatment. Centers for Disease Control and Prevention. May 15, 2024.
  4. West Nile Virus. Johns Hopkins Medicine.
  5. West Nile Virus. Cleveland Clinic. October 16, 2023.
  6. Preventing Mosquito Bites. Centers for Disease Control and Prevention. August 28, 2024.
  7. Current Year Data (2024). Centers for Disease Control and Prevention. January 14, 2025.
  8. Historic Data (1999-2023). Centers for Disease Control and Prevention. February 28, 2025.
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Jane Yoon Scott, MD

Medical Reviewer

Jane Yoon Scott, MD, is an infectious disease physician and an assistant professor of medicine at Emory University in Atlanta. Dr. Scott enjoys connecting with her patients, empowering them to understand and take ownership of their health, and encouraging them to ask questions so that they can make informed and thoughtful decisions.

She graduated with the highest honors from the Georgia Institute of Technology, then received her MD from the Medical College of Georgia. She completed her internal medicine residency training and chief residency at Temple University Hospital, as well as a fellowship in infectious diseases at Emory University. She is board-certified in both internal medicine and infectious diseases.

When she is not seeing patients, Dr. Scott works with neighboring health departments to promote public health, especially to communities that have been historically underserved. She also teaches medical trainees and lectures medical students at the Emory University School of Medicine.

In her free time, Dr. Scott appreciates a good coffee shop, weekend hikes, playing guitar, strolling through cities, sampling restaurants, and traveling to new places.

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Ajai Raj

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Ajai Raj is a reporter specializing in medical science, health, and technology. His work has appeared in Popular Science, Scientific American Mind, The Scientist, and the New York Daily News, as well as a number of medical trade journals.
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