What is West Nile virus (WNV)?
West Nile virus is an illness caused by mosquito bites from infected mosquitoes. Mosquitoes can get West Nile virus from feeding on infected birds.
Each year, approximately 2,000 human cases of West Nile virus are confirmed in the United States. It is the leading cause of illness spread by mosquitoes in the United States.
How does West Nile virus spread? What is considered an outbreak?
The virus spreads through the bites of infected mosquitoes, typically during mosquito season (June through October in most states). While human-to-human transmission is extremely rare, West Nile virus can also spread through blood transfusion and organ transplant if the donor is infected. All donated blood is tested for West Nile virus, and any infected blood is removed from the supply. As of June 2026, organ donors are tested for infection from July 1 through October 31, which is peak season for West Nile virus.
As of July 2026, 18 states have reported West Nile virus cases this year, and that number is expected to grow during the peak mosquito season in August. West Nile virus is a reportable disease. State and territorial health departments report all human cases to the Centers for Disease Control (CDC) to monitor for sustained, high levels of West Nile virus cases in a region.
What are the symptoms of West Nile virus?
About 80% of people infected with West Nile Virus do not have any symptoms. For those who do show symptoms of infection, they typically appear 2 to 6 days after being bitten and can include:
- Fever
- Headache
- Body aches
- Vomiting
- Diarrhea
- Rash
Those with mild to no symptoms typically recover completely, but some symptoms, such as weakness and fatigue, can continue for weeks or months.
Of the 2,000 annual confirmed cases of West Nile virus in the United States, fewer than 1% develop severe symptoms. These more severe symptoms can include high fever, neck stiffness, confusion, and muscle weakness, as well as overall impacts on the central nervous system. People over 60 years old and people with certain medical conditions are at the greatest risk of getting serious symptoms from West Nile virus. If you have any of these symptoms after getting a bug bite, talk to a healthcare professional immediately.
How can individuals and communities prevent West Nile virus?
People can protect themselves by taking preventative measures against mosquito bites, including:
- Using EPA-registered mosquito repellent
- Wearing long, loose-fitting shirts and pants outside
- Avoiding being outside during the hours when mosquitoes are most active, dawn (5:00-8:00 AM) and dusk (5:00-8:00 PM)
- Using screens on open windows and doors
- Draining standing water (like pots that hold water, birdbaths, and pet bowls), where mosquitoes tend to lay their eggs
Some communities may use mosquito control professionals in areas where mosquitoes are infected with the West Nile virus. These professionals may spray insecticides to kill mosquito larvae and adult mosquitoes, reducing the spread of West Nile virus.
Is there a treatment for those infected with West Nile virus?
There are no treatments for West Nile Virus or vaccines to prevent infection. The best way to protect yourself from West Nile is to prevent mosquito bites.
If a healthcare professional diagnoses you with West Nile virus and you have mild symptoms, they may recommend over-the-counter medication to treat fever and pain, hydration, and rest.
Sources:
Centers for Disease Control and Prevention. (2026, June 30). West Nile virus: Current year data (2026). https://www.cdc.gov/west-nile-virus/data-maps/current-year-data.html
Centers for Disease Control and Prevention. (2025, September 10). About West Nile. https://www.cdc.gov/west-nile-virus/about/index.html
Centers for Disease Control and Prevention. (2024, May 15). West Nile and blood transfusions. https://www.cdc.gov/west-nile-virus/causes/blood-transfusions.html
World Health Organization. (2017, October 3). West Nile virus [Fact sheet]. https://www.who.int/news-room/fact-sheets/detail/west-nile-virus
