What Is West Nile?

West Nile virus is a mosquito-borne illness that can cause anything from no symptoms at all to a serious infection of the brain and nervous system. It is the leading cause of mosquito-borne disease in the continental United States. Most people who get infected never feel sick, but a small percentage develop severe, sometimes fatal, illness.

How Do People Get West Nile Virus?

West Nile virus spreads through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which carry the virus in their blood. The infected mosquito can then pass the virus to humans and other animals when it bites them.

You cannot catch West Nile virus from casual contact with an infected person. There is no evidence that the virus spreads through coughing, sneezing, or touching. It also does not spread through sexual contact in any meaningful way, though a few rare cases have been reported through blood transfusion and organ transplantation.

The virus is seasonal. In the United States, transmission typically begins in the summer and continues into the fall until colder weather kills the mosquitoes. Peak transmission usually occurs in August and September.

What Are the Symptoms of West Nile Virus?

Roughly 8 out of 10 people infected with West Nile virus develop no symptoms at all. These people never know they were infected.

About 1 in 5 infected people develop a mild illness called West Nile fever. Symptoms typically appear 2 to 14 days after the mosquito bite and may include:

• Fever
• Headache
• Body aches
• Joint pain
• Vomiting
• Diarrhea
• Skin rash

Most people with West Nile fever recover completely, but fatigue and weakness can linger for weeks or even months.

Less than 1 in 150 infected people develop a severe neuroinvasive form of the disease. This means the virus has entered the brain or the tissues surrounding it. Severe symptoms include high fever, stiff neck, disorientation, coma, tremors, seizures, and paralysis. This form of the disease can be fatal.

Who Is at Highest Risk for Severe Illness?

Age is the strongest risk factor. People over 60 years old are at the highest risk of developing severe neuroinvasive disease if they become infected.

Certain medical conditions also increase risk. People with cancer, diabetes, high blood pressure, kidney disease, or a weakened immune system face a higher chance of severe illness. Organ transplant recipients and people taking immunosuppressive medications are also at greater risk.

Being outdoors in mosquito-heavy areas increases your exposure risk, but it does not necessarily increase your risk of severe disease if infected. The risk of severe disease is driven by biology, not behavior.

How Is West Nile Virus Diagnosed?

Doctors diagnose West Nile virus based on symptoms and a blood test. The blood test looks for antibodies that your immune system produces in response to the infection.

Because symptoms overlap with many other viral infections, testing is usually reserved for people with severe symptoms or those who live in or recently traveled to areas with known West Nile activity. In cases of suspected neuroinvasive disease, a spinal tap may be performed to examine the cerebrospinal fluid.

If you think you have West Nile virus, contact a healthcare provider. Mild cases rarely need specific testing. Severe symptoms like confusion, severe headache, or a stiff neck require immediate medical attention.

What Is the Treatment for West Nile Virus?

There is no specific antiviral medication for West Nile virus. Treatment is supportive, meaning doctors manage the symptoms while your immune system fights the infection.

For mild cases, treatment focuses on rest, fluids, and over-the-counter pain relievers for fever and body aches. Most people recover at home without medical intervention.

Severe cases require hospitalization. Patients may need intravenous fluids, pain management, breathing support, and nursing care to prevent complications. In some cases, physical therapy is needed to regain strength and function after the acute illness passes.

No vaccine for West Nile virus is currently available for humans. Several vaccine candidates have been studied, but none has been approved for use in people.

How Can You Prevent West Nile Virus?

Prevention focuses on avoiding mosquito bites. The most reliable strategies are well established and widely recommended.

Use insect repellent. Products containing DEET, picaridin, IR3535, or oil of lemon eucalyptus are effective. Follow the label instructions carefully. For children, choose a repellent appropriate for their age and apply it sparingly.

Wear protective clothing. Long sleeves, long pants, and socks reduce exposed skin. Treating clothing with permethrin provides additional protection, but do not apply permethrin directly to skin.

Limit outdoor activity during peak mosquito hours. Mosquitoes that carry West Nile virus are most active from dusk to dawn. If you are outside during these hours, take extra precautions.

Mosquito-proof your home. Install or repair screens on windows and doors. Eliminate standing water around your property where mosquitoes breed. Common breeding sites include bird baths, flower pots, clogged gutters, and old tires. Empty and change water in these containers weekly.

Community mosquito control programs also play a role. Many local governments monitor mosquito populations and spray insecticides when West Nile activity is detected. These programs are most effective when residents report dead birds, which can be an early sign of viral activity.

Is West Nile Virus the Same as Other Mosquito-Borne Diseases?

West Nile virus is often grouped with other mosquito-borne viruses like Zika, dengue, and chikungunya. They share a transmission route, but they are different viruses with different behaviors.

Zika and dengue are primarily transmitted by Aedes mosquitoes, which bite during the day. West Nile virus is transmitted mainly by Culex mosquitoes, which bite from dusk to dawn. The symptoms, risk profiles, and long-term outcomes also differ significantly.

West Nile virus is more likely than Zika to cause severe neurological disease in older adults. Zika gained attention for its risk to pregnant women and fetal development, a risk that is not associated with West Nile virus. Understanding which virus you may have been exposed to matters for prognosis and management.

Can You Get West Nile Virus More Than Once?

No. Infection with West Nile virus is thought to provide lifelong immunity. If you recover from the infection, your immune system produces antibodies that protect you from future infection with the same virus.

This immunity is specific to West Nile virus. It does not protect you from other mosquito-borne viruses like dengue or Zika. The practical implication is that people who have already had West Nile virus do not need to change their long-term risk assessment, though avoiding mosquito bites remains important for other diseases.

What Is the Outlook for People Who Get Severe West Nile Disease?

The outlook depends heavily on the severity of the infection. People with mild West Nile fever generally recover fully.

For people with neuroinvasive disease, recovery is slower and less complete. Studies have shown that many survivors experience persistent symptoms for months or years, including fatigue, memory problems, muscle weakness, and difficulty concentrating. The risk of death is highest in older adults and people with underlying medical conditions.

Among people hospitalized with neuroinvasive West Nile disease, the fatality rate is significant, though exact figures vary by outbreak and population. Recovery from the most severe forms can take a year or longer, and some neurological deficits may be permanent.