The 5 mosquito-borne diseases you’re most likely to get in the U.S. (Kathleen Felton, WaPo, June 23, 2026)

St. Johns County voters: I would be honored to have your vote on or before November 3, 2026 for the elected board of our Anastasia Mosquito Control District of St. Johns County, seat 2.   Control of deadly pestiferous mosquitoes helps save lives and protects our tourism and agricultural economy. 

From  The Washington Post, June 23, 2026, 6 AM:

The 5 mosquito-borne diseases you’re most likely to get in the U.S.

These diseases usually cause mild symptoms (or none at all), but they can have more serious complications in certain people.

June 23, 2026 at 6:00 a.m. EDTToday at 6:00 a.m. EDT

9 min

 

(Olivier Morin/AFP/Getty Images)

By Kathleen Felton

Mosquito bites are usually an itchy, swollen nuisance before healing within a few days (longer if you scratch). But occasionally, they can cause bigger problems. Certain mosquito species in the United States can spread infectious organisms when feeding on humans, and those viruses or parasites may make you sick.

These diseases usually cause mild symptoms (or none at all), but they can have more serious complications in certain people. Worldwide, vector-borne diseases, such as those spread by mosquitos and ticks, are responsible for 700,000 deaths a year, according to the World Health Organization.

Mosquitoes can’t regulate their own body temperature and thrive in warm, humid weather, said Erin Gilmour Mordecai, an associate professor of biology at Stanford University who researches the ecology of infectious disease. Considering this, climate change may be extending the seasons when these insects can breed, while also enabling certain species to expand their range, experts said. Additionally, travelers can inadvertently start outbreaks if, once returning home, a local mosquito bites them while they’re still infected.

“Together, all these things are increasing the risk of mosquito-borne diseases in places like the United States, where in the past, maybe it was too cool for transmission,” said Mordecai. 

Below are the mosquito-borne diseases you’re most likely to encounter in the U.S., where these viruses and parasites tend to circulate, and the symptoms to be aware of.

Where: Across the contiguous U.S.

West Nile virus is a leading cause of mosquito-borne disease in the U.S., and it’s important for people to be aware of it, said Eva Buckner, an assistant professor and state extension specialist at the UF/IFAS Florida Medical Entomology Laboratory. The virus is transmitted primarily by Culex mosquitoes, a species that can be found around the country. “Humans get infected with West Nile when you get unlucky and get bitten by the same mosquito that’s bitten an infected bird in the past,” said Mordecai. (Birds are the natural host for the virus.)

Around 2,000 cases are reported each year in the U.S., usually in July, August or September, “but that’s probably an underestimation, because a lot of the time there are no symptoms or super, super mild symptoms,” noted Buckner. Though some people never realize they have West Nile virus, others experience flu-like illness such as a headache, fever and body aches within days of a mosquito bite. 

These symptoms often resolve on their own, but “the concerning condition with West Nile is what we call neuroinvasive West Nile,” said Norman Beatty, an associate professor of medicine in the division of infectious diseases and global medicine at the University of Florida College of Medicine. This can occur when, rarely, the virus travels to the brain and causes inflammation, leading to complications such as meningitis or encephalitis.

Elderly people and those with underlying conditions or weakened immune systems are more at risk for serious complications from West Nile virus. Experiencing neck stiffness, confusion, tremors and a high fever are all urgent signs to reach out to your doctor.

Eastern equine encephalitis

Where: Usually in Eastern or Gulf Coast states 

Eastern equine encephalitis is spread by a few mosquito species, generally in swampy areas around the Gulf Coast and the Eastern U.S. Like West Nile, this virus circulates between humans and birds. While not nearly as common — there were only 218 cases reported between 2003 and 2025 — “when cases do occur, they’re often more severe,” said Buckner.

Some people have a fever, headache and fatigue, but others develop a neurologic disease that leads to meningitis, encephalitis, behavioral changes or seizures. For those with more severe symptoms, “the fatality rate is 30 percent, and people who survive have long-term consequences,” said Laura Harrington, a professor of entomology at Cornell University.

Jamestown Canyon

Where: Usually in the upper Midwest 

First isolated from mosquitoes in Jamestown, Colorado, this virus affects northern areas of the U.S., particularly Midwestern states such as Wisconsin and Michigan. It cycles between different types of mosquitoes and animals, often deer. As with West Nile, most people infected with Jamestown Canyon have flu-like symptoms, but it’s possible to develop severe complications, including encephalitis.

There have been only 362 confirmed cases of Jamestown Canyon virus between 2011 and 2025, but cases seem to be trending upward. “We don’t really understand why it’s increasing,” said Harrington, adding that it could be due to a combination of awareness and improved diagnostic testing.

La Crosse encephalitis virus

Where: Usually in the upper Midwestern, mid-Atlantic and southeastern U.S. 

Like many mosquito-borne diseases, La Crosse causes initial flu-like illness, with some people developing encephalitis. It’s spread between mosquitos and small mammals such as chipmunks. People 16 and under are most at risk for experiencing severe complications. This virus “primarily affects children,” said Buckner.

According to the Centers for Disease Control and Prevention, mild cases are typically under-reported and under-diagnosed, though there are 30 to 90 cases of more serious neuroinvasive La Crosse reported each year.

There’s some concern this virus could become more widespread, said David Hamer, an infectious disease physician and professor of global health and medicine at Boston University School of Public Health. Though currently spread by a mosquito species known as Aedes triseriatus, La Crosse could potentially be transmitted by another vector, Aedes albopictus, which has expanded its range, according to a 2024 paper Hamer co-authored.

St. Louis encephalitis

Where: Eastern, central and southern U.S., as well as some Western states

From the same group of viruses as West Nile, St. Louis encephalitis is also transmitted by Culex mosquitoes and cycles between these insects and birds. Most people have less severe symptoms such as a fever, headache and nausea, but some develop complications, including encephalitis or meningitis.

St. Louis encephalitis is most frequently identified in Central and Eastern states, but there have been reported cases in the South, where warmer temperatures may allow the disease to spread year-round, as well as in California, Arizona and Nevada. There were 1 to 49 cases a year between 2003 and 2025. “We haven’t really had an outbreak of St. Louis encephalitis in about 20 years, but sporadic outbreaks can still occur, so it’s good to be aware of it,” said Buckner. 

Dengue, chikungunya, malaria and Zika virus

It would be unlikely to contract these viruses within the U.S. “Most cases are absolutely going to be occurring as a result of travelers getting infected,” said Buckner. But they’re still important to be aware of, since all can be serious and local transmission, while rare, is possible.

alaria, a potentially deadly mosquito-borne disease caused by a parasite, was once more common in the U.S. There are still about 2,000 cases in this country every year, primarily in people who were infected while traveling. Malaria may be prevented by taking certain medications before traveling to areas where it’s endemic, and a vaccine is available for children living in regions where it circulates regularly.

Dengue, chikungunya and Zika viruses are all spread by Aedes aegypti, a tropical-dwelling mosquito. Dengue, in particular, is a serious global concern: It causes up to 400 million infections and 40,000 deaths each year, and some experts worry transmission in the U.S. may become more common as a result of warming conditions.“We’ve had an unusually high number of locally transmitted cases of dengue virus in the U.S. in recent years, especially in California, Florida and Texas,” noted Harrington.

Since there aren’t targeted treatments for most of these conditions — you have to wait it out and use supportive care until you get better — it’s important to take precautions to avoid bites, such as by using EPA-approved repellents, wearing long clothing and eliminating standing water around your property.

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