About 80% of people infected with West Nile virus never develop any symptoms at all. Of the roughly 20% who do get sick, the first signs typically appear 2 to 6 days after a mosquito bite and look a lot like the flu: sudden fever, headache, and body aches. Because these early symptoms overlap so heavily with other common illnesses, many people never realize they had West Nile.
The Most Common Early Symptoms
The mild form of the illness, called West Nile fever, produces a cluster of symptoms that can easily be mistaken for influenza or another seasonal virus. The hallmark first signs include fever, headache, body aches, and joint pain. Some people also develop vomiting, diarrhea, or a rash. One key difference from a typical cold or flu: West Nile rarely causes significant respiratory symptoms like cough, sore throat, or nasal congestion. If you develop a fever with body aches during mosquito season but without the usual upper respiratory symptoms, that pattern is worth noting.
Most people with this mild form recover completely, but the aftermath can linger. Fatigue and weakness commonly persist for weeks or even months after the initial illness resolves, which can be frustrating when the infection itself seemed minor.
The Rash
Some people with West Nile fever develop a skin rash, though it doesn’t appear in every case. The rash is listed among the general flu-like symptoms of the mild illness but isn’t as consistently present as fever or headache. When it does appear, it typically shows up on the trunk and can be subtle enough to miss. The rash alone isn’t distinctive enough to confirm West Nile, but combined with fever and body aches during peak mosquito months (generally summer and early fall), it adds to the picture.
Warning Signs of Severe Disease
Less than 1% of infected people develop neuroinvasive disease, where the virus crosses into the brain or spinal cord. But this severe form is responsible for the vast majority of serious outcomes. In 2025, over 1,400 of the roughly 2,000 reported cases in the U.S. were neuroinvasive, reported across 47 states. That ratio reflects the fact that mild cases often go undiagnosed and unreported, while the severe ones land people in hospitals.
The signs that West Nile is progressing beyond a mild illness include:
- High fever that is more intense than the moderate fever of mild West Nile
- Severe headache and neck stiffness, which can indicate inflammation of the membranes surrounding the brain and spinal cord
- Disorientation or stupor, signaling that the virus is affecting the brain itself
- Tremors or convulsions
- Sudden muscle weakness, particularly if it’s asymmetric (affecting one side more than the other)
These symptoms can develop rapidly. Someone who initially seemed to have a standard fever may deteriorate within days into confusion, difficulty moving, or even coma. The three forms of severe disease involve inflammation of the brain, inflammation of the protective membranes around the brain and spinal cord, or both.
Paralysis: A Polio-Like Complication
One of the more alarming complications of West Nile is a sudden, polio-like paralysis. It typically strikes as an acute, asymmetric weakness, meaning one limb or one side of the body becomes weak or paralyzed while the other side may be unaffected. Unlike some other causes of paralysis, this form usually involves no pain or sensory loss. You may lose the ability to move a limb but can still feel it being touched.
This happens because the virus attacks the motor nerve cells in the spinal cord, the same cells that poliovirus targets. The weakness tends to come on suddenly and can be severe. Recovery varies widely, and some degree of paralysis may be permanent.
Who Faces the Highest Risk
Anyone bitten by an infected mosquito can contract West Nile, but the risk of developing severe neuroinvasive disease rises sharply with age. People over 60 are significantly more likely to progress from mild symptoms to brain or spinal cord involvement. Those with weakened immune systems, whether from medications, organ transplants, or chronic illness, face a longer incubation period (the window can stretch to 14 days or more) and a higher chance of serious complications.
How West Nile Is Diagnosed
Because the early symptoms look so much like other viral illnesses, a blood test is the only way to confirm West Nile. The standard test looks for specific antibodies your immune system produces in response to the virus. These antibodies typically become detectable 3 to 8 days after symptoms begin and remain present for one to three months.
Timing matters. If you’re tested very early, within the first week of feeling sick, the antibodies may not have built up enough to register. A negative result in that window doesn’t rule out West Nile, and a repeat test a few days later may be needed. A different class of longer-lasting antibodies can persist for years, so their presence alone only proves you were infected at some point, not that a current illness is caused by West Nile.
There is no specific antiviral treatment for West Nile virus. Mild cases are managed with rest, fluids, and over-the-counter pain relief. Severe neuroinvasive cases require hospitalization for supportive care, including help with breathing and intravenous fluids, while the body fights the infection.