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West Nile Virus in the United States

West Nile virus is the leading cause of mosquito-borne disease in the contiguous United States. Culex mosquitoes carry it after feeding on infected birds. Most infections cause nothing at all: the CDC puts it at 80 percent with no symptoms, about 20 percent with a fever, and less than 1 percent with severe illness affecting the central nervous system.

West Nile virus is the leading cause of mosquito-borne disease in the contiguous United States. It cycles between birds and Culex mosquitoes, and people are incidental hosts. The CDC reports that 80 percent of infected people develop no symptoms, and no vaccine or specific medicine exists. Everything below traces to CDC and EPA sources.

What is West Nile virus?

West Nile virus is "a member of the family Flaviviridae, genus Flavivirus," per the CDC. Surveillance authors note it "was first detected in the Americas in 1999."

The CDC's headline numbers: "Each year, about 2,000 people in the United States are diagnosed with West Nile, but this number is an underestimate as most people do not develop symptoms or attribute mild symptoms to other causes." The same page adds: "Each year, more than 1,300 people develop severe illness that affects the central nervous system and more than 130 people die."

West Nile is a nationally notifiable condition, reported to the CDC through a system called ArboNET. As of August 4, 2026, the CDC's current-year dashboard listed a preliminary 143 cases for 2026, including 99 neuroinvasive cases, across 23 states.

Which mosquitoes spread West Nile virus, and how?

The CDC describes the natural cycle plainly: "In nature, West Nile virus cycles between mosquitoes (especially Culex species) and birds." Mosquitoes pick up the virus from infected birds, and "after about a week, infected mosquitoes can spread the virus to more birds when they bite." Surveillance authors name the vectors: the cycle "primarily involves Culex species mosquitoes, particularly Cx. pipiens, Cx. tarsalis, and Cx. quinquefasciatus."

The CDC calls people "dead-end hosts because unlike birds, they do not develop high enough levels of virus in their bloodstream to spread the virus to uninfected mosquitoes that bite them."

Culex behavior explains the exposure pattern. Per the CDC, adult females lay eggs on "permanent standing water like ponds, edges of lakes, swamps, unmaintained swimming pools," plus containers around the home. Adults "are most active from dusk to dawn." Some "prefer to live near and bite birds," biting people "when other animals are not nearby."

Bites do most of the work, but they are not the only route. The virus "can also spread through blood transfusion and organ transplantation," per the CDC, and rarely from mother to baby. It does not spread "through coughing, sneezing, or touching."

What are the symptoms of West Nile virus?

Symptoms usually start 2 to 6 days after an infected mosquito bites, per the CDC, but can start anywhere from 2 to 14 days, and longer in people with a weakened immune system. Then a three-way split, straight from the CDC's symptoms page:

  • No symptoms. "Most people (80%) infected with West Nile virus do not develop symptoms."
  • Mild illness. "About 20% of people who are infected develop flu-like symptoms, such as a fever with headache, body aches, joint pains, vomiting, diarrhea, or rash." Most recover completely, "but fatigue and weakness can last for weeks or months."
  • Severe illness. "Less than 1% of people infected develop severe illness that affects the central nervous system, requires hospitalization, or results in death."

That last percent is the whole reason West Nile matters. The CDC says central nervous system infection "can cause inflammation of the brain (encephalitis) and the membranes around the brain and spinal cord (meningitis)," with symptoms including "high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis." And "about 10% of people who develop severe illness affecting the central nervous system die."

In CDC surveillance for 2009 through 2018, 94 percent of the 12,835 reported neuroinvasive cases were hospitalized. The CDC's trigger line: "Seek immediate medical attention if you experience high fever, neck stiffness, muscle weakness, confusion, or tremors."

Who is at higher risk of severe illness?

Age is the big one. The CDC: "The risk of severe illness increases with age. For example, people 65 and older are three times as likely to develop neurologic illness than people younger than 65." Based on blood donor studies, its clinical page adds, about 2 percent of infected people 65 and older go on to develop neuroinvasive disease, against under 0.5 percent of infected people younger than 65.

The other risk factors are "certain chronic medical conditions like cancer, diabetes, high blood pressure (hypertension), or kidney disease," and a weakened immune system, including from medications for cancer, autoimmune conditions, or preventing organ rejection.

Outcomes track age just as sharply. Case-fatality among neuroinvasive patients was 2 percent under age 50, 6 percent at 50 to 69, and 21 percent at 70 and older. Recovery is often incomplete: the CDC reports 30 to 40 percent of hospitalized patients discharged to long-term care or rehabilitation, and more than 50 percent still having symptoms over a year later.

Where and when does West Nile spread in the United States?

Geographically, essentially everywhere in the lower 48. The CDC: "All states in the contiguous United States have reported cases of West Nile." During 2009 through 2018, 1,580 of 3,142 U.S. counties reported at least one neuroinvasive case.

Risk concentrates. Over that decade, "the highest average annual incidence of neuroinvasive disease occurred in North Dakota (3.16 cases per 100,000 population), South Dakota (3.06), Nebraska (1.95), and Mississippi (1.17)." The largest raw case counts came from California, Texas, Illinois, and Arizona, and six counties inside those four states accounted for 23 percent of neuroinvasive cases nationally.

Seasonally, the window is tight. The CDC: "People get infected with the virus during mosquito season, which starts in the summer and continues through fall (typically June through October). The number of people diagnosed with West Nile typically peaks in late August to early September." Surveillance agrees: 89 percent of patients in 2009 through 2018 had illness onset during July through September. Totals still swing, since "the annual number of reported West Nile cases can fluctuate widely, as a result of periodic epidemics."

One caveat on every count above: the CDC flags that mild cases "are more likely to be underreported compared to more severe disease (neuroinvasive) cases." Surveillance authors put it at 30 to 70 nonneuroinvasive cases for every neuroinvasive case reported, making the 9,034 mild cases reported only 1 to 2 percent of those expected.

Is there a vaccine or treatment for West Nile?

No, on both counts. The CDC states: "No licensed vaccines or medicines are available to prevent West Nile virus disease (West Nile) in people." Its clinical guidance is equally blunt: "No treatment is available for West Nile virus (WNV) disease. Clinical management is supportive." Various drugs have been evaluated, but "none has shown conclusive benefit to date."

Care is symptom management. For mild illness the CDC suggests over-the-counter medication like acetaminophen, hydration, and rest, while avoiding NSAIDs if you live in an area with dengue. "Antibiotics do not treat viral infections." Severe illness often requires hospitalization for "intravenous fluids, pain medication, and nursing care." One piece of good news: "Most people infected with West Nile virus are believed to have lifelong immunity."

How does the CDC say to prevent West Nile?

With no vaccine, the CDC's position is that "the best way to protect yourself and your loved ones from West Nile is to prevent mosquito bites." Its recommendations:

  • "Use an insect repellent registered with the Environmental Protection Agency."
  • "Wear long, loose-fitting shirts and pants so it is harder for mosquitoes to bite your skin."
  • "Avoid being outside between dusk and dawn when the mosquitoes that spread West Nile virus are most active."
  • "Use screens on windows and doors or air conditioning, if available, to keep mosquitoes outside."

On repellents, the CDC lists six active ingredients in EPA-registered products: DEET, picaridin (known as KBR 3023 and icaridin outside the United States), IR3535, oil of lemon eucalyptus, para-menthane-diol, and 2-undecanone. It tells people to "look for an EPA registration number (EPA Reg. No.) on the insect repellent product label," which "means the company provided EPA with technical information on the effectiveness of the product." The EPA is explicit that "EPA and the U.S. Government do not endorse any product or service." CDC tick bite guidance names the same six ingredients; our Lyme prevention guide covers that overlap.

The rest of the CDC's mosquito guidance: sunscreen first and repellent second, no oil of lemon eucalyptus or para-menthane-diol on children under 3, permethrin-treated clothing and gear (never on skin), and once a week empty, scrub, cover, or throw out items around the house that hold water. At the community level, "mosquito control professionals may use products to kill mosquito larvae (larvicides) or adult mosquitoes (adulticides)."

What should you do if you think you have West Nile?

Talk to a healthcare provider. The CDC says providers diagnose infection based on signs and symptoms, history of possible mosquito exposure, and laboratory testing, and can test for West Nile virus or other infections that cause similar symptoms.

Two follow-ups people miss. Blood: "people recently diagnosed with West Nile virus infection should not donate blood or bone marrow for 120 days." Dead birds: avoid bare-handed contact with any dead animal, and use "gloves or double plastic bags to place the carcass in a garbage can." Any decision about testing or treatment belongs with your clinician.

Where Longsleeve fits

Longsleeve is an EPA-registered picaridin lotion (EPA Reg. No. 101457-1). It repels mosquitoes and ticks for up to 14 hours. It is fragrance-free, with nine disclosed ingredients.

Common questions

What are the odds of getting seriously ill from West Nile?

The CDC says 80 percent of infected people develop no symptoms, about 20 percent get flu-like symptoms, and less than 1 percent develop severe illness affecting the central nervous system. About 10 percent of those die.

Is there a West Nile vaccine?

No. The CDC states that no licensed vaccines or medicines are available to prevent West Nile virus disease in people. There is also no medicine that treats it. Clinical management is supportive.

Which months carry the most risk?

The CDC says infections happen during mosquito season, typically June through October, with diagnoses peaking in late August to early September. CDC surveillance for 2009 through 2018 found 89 percent of patients had illness onset in July through September.

Can you catch West Nile from another person?

Not through ordinary contact. The CDC says the virus does not spread through coughing, sneezing, or touching. It can spread through blood transfusion and organ transplantation, and rarely from mother to baby. Blood and some organ donors are screened.

Sources

  1. CDC, About West Nile. https://www.cdc.gov/west-nile-virus/about/index.html
  2. CDC, Symptoms of West Nile. https://www.cdc.gov/west-nile-virus/signs-symptoms/index.html
  3. CDC, West Nile: Causes and How It Spreads. https://www.cdc.gov/west-nile-virus/causes/index.html
  4. CDC, Preventing West Nile. https://www.cdc.gov/west-nile-virus/prevention/index.html
  5. CDC, Clinical Signs and Symptoms of West Nile Virus Disease. https://www.cdc.gov/west-nile-virus/hcp/clinical-signs/index.html
  6. CDC, Clinical Treatment and Prevention. https://www.cdc.gov/west-nile-virus/hcp/treatment-prevention/index.html
  7. CDC, Data and Maps for West Nile. https://www.cdc.gov/west-nile-virus/data-maps/index.html
  8. CDC, Current Year Data (2026). https://www.cdc.gov/west-nile-virus/data-maps/current-year-data.html
  9. CDC, Historic Data (1999 to 2025). https://www.cdc.gov/west-nile-virus/data-maps/historic-data.html
  10. CDC, About Culex Mosquitoes. https://www.cdc.gov/mosquitoes/about/culex-mosquitoes.html
  11. CDC, Preventing Mosquito Bites. https://www.cdc.gov/mosquitoes/prevention/index.html
  12. CDC, Preventing Tick Bites. https://www.cdc.gov/ticks/prevention/index.html
  13. McDonald E, Mathis S, Martin SW, Staples JE, Fischer M, Lindsey NP. Surveillance for West Nile Virus Disease, United States, 2009 to 2018. MMWR Surveillance Summaries, 2021;70(1):1-15. https://www.cdc.gov/mmwr/volumes/70/ss/ss7001a1.htm
  14. EPA, Find the Repellent that is Right for You. https://www.epa.gov/insect-repellents/find-repellent-right-you

Last reviewed 2026-08-06 · every claim on this page is sourced above