West Nile virus is a mosquito-borne infection that causes no symptoms in roughly 80% of people who contract it. About 20% develop a fever-like illness, and less than 1% experience severe neurological complications. The virus circulates between mosquitoes and birds in nature, with humans picking it up through the bite of an infected mosquito.
How West Nile Virus Spreads
In the wild, West Nile virus cycles continuously between Culex species mosquitoes and birds. A mosquito feeds on an infected bird, picks up the virus, and passes it to the next animal it bites. When that next bite happens to land on a person, the virus enters the human bloodstream.
Humans are what scientists call “dead-end hosts.” Your body doesn’t build up enough virus in the blood for a mosquito to pick it back up and carry it elsewhere, so you can’t fuel the cycle the way birds do. Horses and other mammals are dead-end hosts too. In rare cases, the virus has spread through blood transfusions, organ transplants, and from mother to baby during pregnancy, delivery, or breastfeeding, but mosquito bites account for the vast majority of infections.
What Infection Feels Like
Most people never know they were infected. The 20% who do develop symptoms typically get what’s called West Nile fever: a sudden onset of fever, headache, body aches, joint pain, and sometimes a skin rash. It’s unpleasant but generally resolves on its own.
The dangerous form of the disease, neuroinvasive West Nile, occurs in fewer than 1 in 100 infected people. The virus crosses into the central nervous system and causes inflammation of the brain (encephalitis) or the membranes surrounding the brain and spinal cord (meningitis). Symptoms at this stage are unmistakable: high fever, severe headache, neck stiffness, disorientation, tremors, convulsions, muscle weakness, vision loss, numbness, or paralysis. Some people become stuporous or slip into a coma. About 10% of those who reach this stage die.
Who Faces the Highest Risk
Anyone bitten by an infected mosquito can develop symptoms, but three factors raise the odds of severe illness: older age, chronic conditions like cancer, diabetes, high blood pressure, or kidney disease, and a weakened immune system. People taking immunosuppressive medications are at particular risk because their bodies are less equipped to contain the virus before it reaches the brain.
How It’s Diagnosed
If your doctor suspects West Nile, the first step is a blood test looking for antibodies your immune system produces in response to the virus. These antibodies typically appear within a few days of symptom onset. In cases involving neurological symptoms, a sample of spinal fluid may be tested as well.
One complication with testing is that West Nile belongs to a family of related viruses, so antibody tests can sometimes cross-react and produce a false signal. When that’s a concern, a more specific follow-up test can distinguish West Nile from its relatives. In immunocompromised patients, doctors may also look for the virus’s genetic material directly, since these patients sometimes produce antibodies more slowly.
Treatment and Recovery
There is no antiviral drug for West Nile and no human vaccine. Treatment is entirely supportive. For West Nile fever, that means rest, fluids, and over-the-counter pain relief. For neuroinvasive disease, hospitalization is typically necessary, with care focused on managing fever, preventing secondary infections, and supporting breathing if muscle weakness affects the chest.
Recovery from the milder form usually takes days to weeks. Recovery from neuroinvasive disease is a different story. Research published in Emerging Infectious Diseases tracked patients for 18 months after their acute illness and found lasting physical, cognitive, and functional problems. More than 40% continued to report difficulty walking, muscle weakness, fatigue, and insomnia a year and a half later. Thirty percent still experienced memory loss, confusion, depression, and irritability. The same proportion needed help with daily activities that required physical strength, like heavy housework or shopping. Every patient diagnosed with encephalitis and muscle weakness in the study still had trouble walking six months out.
How Many People Get It
West Nile is the leading cause of mosquito-borne illness in the continental United States. In 2025, the CDC recorded 2,076 human cases across 47 states, with 1,434 of those classified as neuroinvasive. Because the vast majority of infections cause no symptoms and are never tested for, the true number of people infected each year is far higher than reported case counts suggest.
Preventing Mosquito Bites
Since there’s no vaccine or specific treatment, prevention comes down to avoiding bites. The CDC recommends EPA-registered insect repellents containing one of these active ingredients: DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol, or 2-undecanone. All are considered safe and effective when used as directed, including for pregnant and breastfeeding women.
Beyond repellent, practical steps make a real difference. Wear long sleeves and pants during dawn and dusk, when Culex mosquitoes are most active. Empty standing water from flower pots, gutters, birdbaths, and old tires around your home, since even a bottle cap of stagnant water can serve as a breeding site. Make sure window and door screens are intact. These measures are especially important in late summer and early fall, when West Nile transmission peaks in most of the United States.