West Nile virus is a mosquito-borne infection that causes no symptoms at all in about 80% of people who contract it. Roughly 20% develop a flu-like illness called West Nile fever, and less than 1% experience severe, potentially life-threatening complications involving the brain and spinal cord. First identified in Uganda in 1937, the virus is now found on every continent except Antarctica and remains the leading cause of mosquito-borne disease in the continental United States, with 2,076 human cases reported across 47 states in 2025 alone.
How the Virus Spreads
West Nile virus circulates in nature through a cycle between Culex species mosquitoes and birds. Certain bird species develop high concentrations of the virus in their blood, and when a mosquito feeds on an infected bird, the mosquito picks up the virus. After about a week, that mosquito can pass the virus to new birds with each bite, keeping the cycle going.
Humans get infected when a virus-carrying mosquito bites them, but the chain stops there. People (along with horses and other mammals) are considered “dead-end hosts,” meaning the virus never builds up enough in human blood for another mosquito to pick it up and spread it further. In rare cases, the virus has also been transmitted through blood transfusions, organ transplants, from mother to child during pregnancy, and through breastfeeding.
Symptoms of West Nile Fever
Most people bitten by an infected mosquito never know it happened. Of the roughly 20% who do get sick, symptoms typically appear 2 to 14 days after the bite and resemble a moderate flu: fever, headache, body aches, joint pain, vomiting, diarrhea, or a rash. This is West Nile fever, and it usually resolves on its own within a few days to several weeks, though fatigue and weakness can linger for weeks or even months afterward.
When It Becomes Severe
In fewer than 1 in 150 infections, the virus crosses into the central nervous system. This is called neuroinvasive disease, and it can take three forms: inflammation of the brain (encephalitis), inflammation of the membranes surrounding the brain and spinal cord (meningitis), or sudden weakness or paralysis in the limbs (acute flaccid paralysis). Warning signs include high fever, severe headache, neck stiffness, disorientation, tremors, seizures, muscle weakness, and vision loss.
Neuroinvasive disease is a medical emergency. Of the 2,076 West Nile cases reported in the U.S. in 2025, 1,434 were classified as neuroinvasive, reflecting the fact that milder cases often go undiagnosed and unreported. Some people who survive severe illness recover fully, but others are left with long-term neurological problems, including persistent weakness, memory difficulties, and fatigue that can last months or years.
Who Faces the Greatest Risk
Anyone can develop severe West Nile disease, but certain factors significantly raise the odds. Older age is the strongest predictor. Chronic conditions like cancer, diabetes, high blood pressure, and kidney disease also increase vulnerability. So does a weakened immune system, whether from an underlying condition or from medications that suppress immune function, such as those taken after an organ transplant.
How It’s Diagnosed
Doctors diagnose West Nile by testing blood or spinal fluid for antibodies the immune system produces in response to the virus. These antibodies typically become detectable 3 to 8 days after symptoms begin and remain present for 30 to 90 days, sometimes longer. Because antibodies may not show up immediately, a negative test taken within the first week of illness doesn’t rule out infection, and a second test a couple of weeks later may be needed.
One complicating factor is that West Nile virus belongs to the same family as dengue, Zika, and yellow fever viruses. Antibodies against these related viruses can cross-react on initial tests, so a more specialized follow-up test is sometimes used to confirm which specific virus caused the infection.
Treatment Options
There are no antiviral medications for West Nile virus, and antibiotics don’t work against it. For the majority of people who develop West Nile fever, recovery happens at home with rest, fluids, and over-the-counter pain relievers for headache and body aches.
People with severe neuroinvasive disease often need hospitalization. Treatment is supportive: intravenous fluids, pain management, and nursing care to manage complications like breathing difficulties or seizures. There is currently no vaccine approved for use in humans, though vaccines do exist for horses.
Reducing Your Exposure
Since there is no vaccine or specific treatment, prevention comes down to avoiding mosquito bites. Culex mosquitoes, the primary carriers, are most active from dusk through dawn. Using insect repellent containing DEET, picaridin, or oil of lemon eucalyptus during those hours offers strong protection. Wearing long sleeves and pants when outdoors in the evening helps as well.
Around your home, eliminating standing water is the single most effective step. Culex mosquitoes breed in small pools of stagnant water: birdbaths, clogged gutters, flowerpot saucers, old tires, and even bottle caps. Dumping or refreshing standing water at least once a week disrupts the breeding cycle. Installing or repairing window and door screens keeps mosquitoes out of living spaces, which matters especially during peak mosquito season from late summer through early fall in most of the U.S.