West Nile virus is a mosquito-borne infection that causes no symptoms in most people but can lead to serious, sometimes permanent neurological damage in a small percentage of cases. It belongs to the same family of viruses as dengue and Zika, and it circulates in nature between mosquitoes and birds. Humans are essentially dead-end hosts, meaning they get infected but don’t typically pass the virus along. In 2025, the CDC recorded 2,076 human cases across 47 states.
How the Virus Spreads
West Nile virus lives in a cycle between Culex species mosquitoes and wild birds. Certain bird species develop high levels of the virus in their bloodstream after being bitten by an infected mosquito. When another mosquito feeds on that bird, it picks up the virus and can then pass it to the next animal or person it bites. Humans and horses are incidental hosts: they can get sick, but the virus doesn’t build up enough in their blood to continue the chain.
Most infections happen through mosquito bites during warm months, when mosquito populations peak. In rare cases, the virus has also been transmitted through blood transfusions and organ transplants. It does not spread through casual contact, coughing, or touching an infected person.
What Most People Experience
About 80% of people infected with West Nile virus never feel sick at all. Among those who do develop symptoms, the illness is usually mild. It typically shows up 2 to 6 days after a bite, though the incubation period can range from 3 to 14 days. Roughly 95% of people who get sick will have symptoms within a week of infection.
Mild West Nile fever looks a lot like many other viral infections: fever, headache, body aches, fatigue, and sometimes a skin rash or swollen lymph nodes. Most people recover completely, but fatigue and weakness can drag on for weeks or even months after the initial illness clears.
When It Becomes Serious
Less than 1% of infected people develop neuroinvasive disease, where the virus crosses into the brain and spinal cord. That small percentage, however, accounts for the majority of hospitalizations and long-term harm. In 2025, nearly 70% of the reported cases (1,434 out of 2,076) were neuroinvasive.
Neuroinvasive West Nile disease takes three main forms:
- Meningitis: inflammation of the membranes surrounding the brain and spinal cord, causing fever, severe headache, stiff neck, and sensitivity to light.
- Encephalitis: inflammation of the brain itself, which can cause confusion, altered mental status, tremors, seizures, and movement disorders resembling Parkinson’s disease.
- Acute flaccid myelitis: sudden weakness or paralysis in one or more limbs, caused by the virus attacking the spinal cord. This can progress rapidly to respiratory failure requiring a ventilator.
Recovery from severe disease is slow and often incomplete. Between 30% and 40% of patients hospitalized for West Nile disease are discharged to long-term care or rehabilitation facilities rather than home. More than half still have symptoms a year later. People who recover from encephalitis or paralysis frequently have lasting neurological problems, including difficulty with memory, concentration, walking, or daily tasks.
Who Faces the Highest Risk
Anyone bitten by an infected mosquito can get West Nile virus, but severe illness is not evenly distributed. Older adults, particularly those over 60, face a significantly higher chance of developing neuroinvasive disease. People with weakened immune systems, whether from organ transplants, cancer treatment, or conditions like HIV, are also at elevated risk. Some research suggests that high blood pressure, diabetes, and kidney disease may increase vulnerability, though age remains the strongest predictor.
How It’s Diagnosed
Diagnosing West Nile virus requires a blood test or, in severe cases, a spinal fluid test. Doctors look for antibodies the immune system produces in response to the virus. These antibodies typically become detectable within the first week of illness. One complication is that West Nile is closely related to other viruses in its family, so initial antibody tests sometimes cross-react. When that happens, a more specific follow-up test called a neutralization assay can pin down exactly which virus caused the infection.
For patients with weakened immune systems who may not produce antibodies normally, a different approach using genetic detection (looking for the virus’s RNA directly) can sometimes confirm infection if samples are collected early enough.
Treatment Options
There is no antiviral drug or vaccine for West Nile virus in humans. Treatment is entirely supportive, meaning it focuses on managing symptoms while the body fights the infection. For mild cases, that means rest, fluids, and over-the-counter pain relievers for fever and headache.
Severe cases require hospitalization. Patients with meningitis often need medication to control intense headaches and nausea. Those with encephalitis are monitored for brain swelling, seizures, and breathing problems. Patients with acute flaccid myelitis may need a ventilator if the paralysis affects the muscles used for breathing. Rehabilitation, including physical and occupational therapy, is a major part of recovery for anyone with neuroinvasive disease. Various drugs have been studied over the years, but none has shown clear benefit.
Preventing Infection
Since there’s no vaccine, prevention comes down to avoiding mosquito bites and reducing mosquito breeding grounds. 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, a few practical steps make a real difference. Wear long sleeves and pants during peak mosquito hours (dawn and dusk). Keep window and door screens in good repair. Use air conditioning when possible, since mosquitoes rarely enter well-sealed, cooled spaces.
Eliminating standing water around your home is one of the most effective things you can do. Mosquitoes lay eggs in even small amounts of still water. Once a week, empty and scrub anything that collects water: flower pot saucers, bird baths, buckets, old tires, kiddie pools, and pet water bowls. It takes surprisingly little water for a Culex mosquito to complete its life cycle.