Most people infected with West Nile virus never know they have it. About 8 out of 10 people bitten by an infected mosquito develop no symptoms at all. Of those who do get sick, most experience a moderate flu-like illness that resolves on its own. But a small percentage develop serious neurological disease that can cause lasting damage or, in rare cases, death.
The First Days After Infection
After a bite from an infected mosquito, the virus enters your bloodstream and begins replicating. You won’t feel anything right away. The incubation period is typically 2 to 6 days, though it can stretch to 14 days. People with weakened immune systems may take even longer to show symptoms, if they develop them at all.
West Nile Fever: The Milder Form
When symptoms do appear, they usually take the form of West Nile fever, a non-neuroinvasive illness. This looks a lot like the flu: headache, body aches, fatigue, fever, and sometimes a skin rash or swollen lymph nodes. Some people also experience nausea, vomiting, or diarrhea. The illness is unpleasant but not dangerous for most people, and it typically clears within a few days to a couple of weeks.
There’s no specific antiviral medication for West Nile. Treatment for the milder form is what you’d do for any viral illness: rest, fluids, and over-the-counter pain relievers for fever and aches. Most people recover fully without medical intervention.
Severe Neurological Disease
The real danger comes when the virus crosses from the bloodstream into the brain and spinal cord. This is called neuroinvasive disease, and it takes three main forms: meningitis (inflammation of the membranes around the brain and spinal cord), encephalitis (inflammation of the brain itself), and acute flaccid paralysis (a polio-like weakness in the limbs).
Symptoms of severe disease include high fever, severe headache, neck stiffness, disorientation, stupor, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. Some patients progress to coma. These symptoms require hospitalization, where treatment focuses on managing complications like breathing difficulties and preventing secondary infections. There is no antiviral drug that targets the virus directly.
Among reported West Nile cases in 2023, 68% were classified as neuroinvasive disease, though this number is skewed by the fact that mild cases are far less likely to be reported or even diagnosed. The CDC estimates roughly 2,000 disease cases are reported annually in the U.S., including about 1,200 neurological illnesses and approximately 120 deaths. Illness onset clusters heavily between July and September, peak mosquito season.
Who Faces the Highest Risk
Anyone can develop severe illness, but certain factors raise the odds significantly. Age is the strongest predictor. The median age of reported patients in 2023 was 63, and men made up 63% of cases. Chronic conditions like diabetes, high blood pressure, kidney disease, and cancer also increase risk. People with weakened immune systems, whether from medications or underlying conditions, are especially vulnerable because their bodies may struggle to mount an effective antibody response.
Long-Term Effects After Recovery
Surviving the acute illness doesn’t always mean a full return to normal. This is one of the most underappreciated aspects of West Nile. Between 30% and 60% of people who recover from neuroinvasive disease experience lasting neurological problems. These aren’t minor inconveniences. The most commonly reported long-term symptoms are depression, memory loss, persistent fatigue, and motor dysfunction like weakness or tremors.
The outlook depends heavily on which form of neurological disease you had. Patients who had meningitis tend to recover more completely. Those who had encephalitis fare worse: up to 86% still showed abnormal neurological findings on exams three years after their infection. Memory problems are particularly common, with roughly one in five survivors showing measurable deficits in both short-term and long-term memory.
For those who developed paralysis, the recovery picture is mixed. About a third achieve complete or near-complete recovery, a third show partial recovery, and a third see little to no improvement. Weakness in particular tends to persist even after other symptoms resolve.
Depression is strikingly common among survivors, with studies finding rates of major depressive disorder between 21% and 56% of patients after infection. These psychological effects can persist for months, years, or the rest of a person’s life.
How It Gets Diagnosed
If you develop symptoms consistent with West Nile, especially during mosquito season, diagnosis starts with a blood test looking for antibodies your immune system produces in response to the virus. These antibodies typically become detectable within a few days of symptom onset. In cases of suspected neurological involvement, doctors may also test spinal fluid. For people with compromised immune systems, who may not produce antibodies on a normal timeline, molecular testing that detects the virus’s genetic material can be more reliable.
No Vaccine Exists Yet
Despite being the leading cause of mosquito-borne illness in the United States, there is no licensed vaccine for West Nile virus in humans. At least seven vaccine candidates have entered early-stage clinical trials over the years, but none have progressed to approval. The relatively unpredictable year-to-year case counts and the challenges of designing clinical trials for a seasonal disease have slowed development. Prevention still comes down to avoiding mosquito bites: using repellent, wearing long sleeves during peak hours, and eliminating standing water where mosquitoes breed.