The outbreak, the biggest since Florida counted 58 local cases in 2010, dwarfs the state's previous recent high of 189 locally transmitted infections in 2023. In addition to the homegrown cases, Florida has recorded 382 international travel-associated dengue cases this year, according to state arbovirus surveillance data.

On Nov. 5, low-flying helicopters sprayed a fine mist over parts of Pinellas and Hillsborough counties, dispersing water mixed with Bti — a naturally occurring soil bacterium that kills mosquito larvae — in an effort to slow transmission.

Dengue is caused by any of four distinct dengue viruses and spreads through the bite of infected Aedes mosquitoes. It cannot be passed from person to person, and there is no widely available vaccine. About three in four infected people never develop symptoms, but those who do typically fall ill four to 10 days after the bite, experiencing high fever, headache, nausea, vomiting, rash, eye pain and severe muscle and joint aches.

About one in 20 infected people develops severe dengue, a life-threatening emergency that can cause shock, internal bleeding and death. Warning signs include severe stomach pain, persistent vomiting, bleeding from the gums or nose, rapid breathing, blood in vomit or stool, and extreme fatigue or restlessness. Health officials urge anyone experiencing these symptoms to seek immediate medical care.

There is no specific cure. Most patients recover within about a week with rest, fluids and acetaminophen for fever and pain. The Centers for Disease Control and Prevention advises against aspirin and ibuprofen because they can increase the risk of bleeding.

A prior infection with one dengue virus provides lifelong immunity only to that specific type, leaving people vulnerable to the other three — and a second infection with a different type can be more severe than the first.

"Dengue is a doozy, mainly because the second infection can be more dangerous than the first," epidemiologist Katelyn Jetelina wrote in social media posts.

Kristi Miley, a University of Florida public health professor, said the dengue type predominantly circulating in Florida this year "has been noted to cause more severe symptoms than the other three serotypes." Florida has documented all four dengue viruses since 2009.

Miley said a single infected female mosquito can pass the virus to her eggs, potentially producing more than 500 infected offspring. Combined with heavy late-season rainfall, she said, "one can quickly see how an outbreak of this magnitude can happen."

Isabella White, a spokesperson for the Florida Department of Health, said studies are ongoing to better understand what is driving the state's 2026 dengue activity.

Florida's warm, humid climate provides ideal conditions for Aedes mosquitoes. The state's previous 16 years combined saw only 606 locally acquired cases, making this year's single-season total nearly as large as that entire span.

The risk is not confined to Florida. In 2025, the CDC recorded locally acquired dengue in Florida, California and Arizona, and in August Virginia health officials reported a locally acquired infection in a northern Virginia resident.

To protect against dengue, health officials recommend dumping or covering standing water — especially after rain — discarding old tires and drums, cleaning bird baths and pet bowls at least once or twice a week, and covering boats and vehicles with tarps. Residents should use EPA-registered repellents containing DEET or picaridin and wear long sleeves, pants, socks and shoes. The mosquitoes that carry dengue are aggressive daytime biters.

Anyone who suspects a dengue infection should seek medical care and avoid mosquito exposure for seven days after symptoms begin, the period during which the virus can be passed to biting mosquitoes.