You get West Nile virus from the bite of an infected mosquito. That is, by far, the most common route of infection, and for most people it’s the only realistic way to contract the virus. About 1,800 cases were reported in the United States in 2024 alone, and since tracking began in 1999, more than 61,000 Americans have been diagnosed with the disease.
The Mosquito-Bird Cycle
West Nile virus doesn’t start with people. It circulates between mosquitoes and birds. Certain wild birds, including crows, robins, sparrows, jays, hawks, and owls, can carry extremely high levels of the virus in their blood. When a mosquito bites one of these birds, the mosquito picks up the virus. After about a week of incubation inside the mosquito, that insect can now pass West Nile to the next animal it bites.
Humans, horses, and other mammals are what scientists call “dead-end hosts.” When a mosquito bites you and delivers the virus, your bloodstream never builds up enough virus to infect another mosquito that bites you later. You can’t spread it further. The cycle depends entirely on birds and mosquitoes passing it back and forth, with humans as bystanders who occasionally get caught in the crossfire.
The mosquitoes most responsible for spreading West Nile to people belong to the Culex genus. In North America, Culex pipiens (common in northern states), Culex quinquefasciatus (more common in the South), and Culex salinarius are the primary carriers. These species tend to bite at dusk and dawn, which is when your risk is highest.
When Transmission Risk Peaks
West Nile is a warm-weather disease. Mosquitoes need standing water to breed and warm temperatures to be active, so transmission in the U.S. peaks during summer and early fall. Most cases occur between June and October, with the highest numbers typically in August and September. States with hot summers and areas that get heavy rainfall tend to see larger outbreaks because both conditions favor mosquito populations.
Geography matters too. West Nile has been detected in all 48 contiguous states, but case counts vary widely year to year depending on local weather, mosquito control efforts, and bird migration patterns.
Rare Ways the Virus Spreads
Mosquito bites account for the vast majority of infections, but a small number of cases have occurred through other routes. West Nile virus has been transmitted through organ transplantation when a donor had an active infection at the time of donation. The risk is low, but transplant recipients face a disproportionate threat: 75% of transplant recipients who contract West Nile develop severe illness, largely because the immune-suppressing medications they take leave them vulnerable.
Blood transfusions were another concern in the early 2000s, which led to routine screening of the blood supply. Transmission from mother to baby during pregnancy is possible but rare. Only a handful of cases in newborns have ever been reported, and pregnant women are not at higher risk of getting infected in the first place.
You cannot get West Nile virus from casual contact with an infected person. Touching, kissing, or caring for someone with the disease poses no risk.
What Happens After You’re Infected
If an infected mosquito bites you, symptoms typically appear within 2 to 6 days, though the window can stretch to 14 days. People with weakened immune systems may take even longer to show signs of illness.
The reality is that most people never know they were infected. About 80% of those who contract West Nile virus have no symptoms at all. Of the remaining 20%, most develop what’s called West Nile fever: headache, body aches, fatigue, joint pain, vomiting, diarrhea, or a rash. It’s unpleasant but typically resolves on its own.
The serious concern is neuroinvasive disease, which affects roughly 1 in 150 infected people. This occurs when the virus crosses into the brain and spinal cord, causing encephalitis (brain inflammation) or meningitis (inflammation of the membranes surrounding the brain). Symptoms include high fever, neck stiffness, disorientation, tremors, seizures, and muscle weakness. Some of these effects, particularly muscle weakness, can be permanent. Since 1999, over 3,100 deaths in the U.S. have been attributed to West Nile virus, nearly all from neuroinvasive disease. Older adults and people with compromised immune systems face the greatest risk of severe outcomes.
How to Reduce Your Risk
There is no vaccine for West Nile virus in humans and no specific treatment for the infection. Prevention comes down to avoiding mosquito bites, especially during peak season.
Insect repellents are the most effective personal defense, but not all repellents work equally well. Only products containing EPA-registered active ingredients have been scientifically proven to keep mosquitoes away. The three with the strongest evidence are DEET, picaridin (sometimes labeled icaridin), and oil of lemon eucalyptus (the active compound is called PMD). DEET is available in roughly 120 formulations from about 30 companies and is considered safe for all ages. Picaridin is often sold at lower concentrations than DEET, which means you’ll need to reapply it more frequently. Higher concentrations of any active ingredient last longer per application but can feel stickier on the skin. These repellents are also considered safe for pregnant and breastfeeding women.
Beyond repellent, simple environmental steps make a difference. Dump any standing water around your home, since even a bottle cap’s worth can serve as a mosquito breeding site. Flower pot saucers, clogged gutters, old tires, and birdbaths are common culprits. Wear long sleeves and pants during dusk and dawn when Culex mosquitoes are most active. Window and door screens in good repair keep mosquitoes out of your home, where they can bite you while you sleep.