In the past, Americans were infected with dengue after being bitten by mosquitoes while traveling. But now, most of the current transmission is from local mosquitoes.

On the last Friday in August, Gay Rabb Small went for her daily walk along the Tampa shore. Nearing 80, she was determined to stay healthy and walked at least five miles a day.

That night at dinner, her daughter, Anna Small, noticed that her mother seemed a little off. “I thought she had overdone it,” Ms. Small said.

By Saturday, Gay Small had developed a tremor, and her daughter took her to the emergency room, where she was diagnosed with dengue, a viral illness transmitted by mosquitoes. She died the following Friday, Sept. 4, her 80th birthday.

Gay Small got dengue from a mosquito bite, probably from one of her daily walks. She is so far the only fatality among hundreds of people in Florida who have been sickened in a dengue outbreak that scientists and public health officials are struggling to understand. The size of the outbreak, they say, indicates that dengue may be making a comeback in the continental United States at a level not seen for many decades.

“I think the dengue outbreaks will get worse over the coming years in the U.S.,” said Ira Longini, a biostatistician at the University of Florida.

In the continental United States, dengue cases are almost always traceable to mosquito bites from traveling. So it is especially foreboding, public health experts say, that most of the current transmission in Florida is coming from mosquito bites locally.

“It is very unusual to have transmission in the United States,” said Eva Harris, a professor of infectious diseases and vaccinology at the University of California, Berkeley. “This is so very much of an eye-opener.”

Most people infected by dengue show no symptoms, so they don’t even realize that they have it. But the illness can be very painful, causing high fever, joint and muscle aches, headaches and pain behind the eyes. Most patients with severe symptoms recover, but in a small number of cases, dengue can be fatal, leading to shock, hemorrhage and organ failure.

About 2,678 people have been infected with dengue in the United States and its territories so far this year, according to the Centers for Disease Control and Prevention. Almost half the cases have been in Puerto Rico and American Samoa, where dengue is endemic.

But Florida is by far the hardest-hit state, with 880 cases. About 503 of those people were infected in the United States, up from a previous high of 189 people infected locally in 2023, according to the C.D.C. In 2025, Florida had 517 cases, and only 72 of them were infected locally, according to the C.D.C.

This year, the center of the outbreak moved from Miami-Dade County, with 254 cases, to Hillsborough County, which encompasses Tampa, with 417 cases. Of people who caught the virus where they lived, 395 were in Tampa and just 12 in Miami.

Public health officials are puzzling over what is causing the shift from southern to central Florida and why local transmission has risen.

“That is the $100 million question,” said Dr. Gabriela Paz-Bailey, chief of the dengue branch in the C.D.C.’s division of vector-borne diseases. “Maybe it is associated with higher introductions through travelers to the Caribbean. But we’re still trying to work out why we’re seeing more cases this year."

Dr. Paz-Bailey said she did not expect to see “explosive” outbreaks like in Puerto Rico because of the higher standard of living in the continental United States. “But I do think that we will continue to see locally acquired dengue — that this will become the norm,” she said.

There are some forces that are probably contributing to the dengue outbreak in Florida. Warm, wet weather helps mosquitoes breed. Torrential rains have flooded parking lots and playgrounds lately in southern Florida, leading health authorities to fear that mosquitoes will proliferate.

The natural habitat of the mosquitoes that carry dengue is in tropical and subtropical countries, so climate change may be a factor. Florida is also a major transit hub for tourists and other travelers to the Caribbean and Latin America, who may bring the virus back with them. Mosquitoes may stow away on boats and planes.

In recent years, local transmission has been recorded where it was scarce before, in Texas, Arizona, California and with one case this year, Virginia. Dengue attributed to travel has been reported as far north as Minnesota and Maine.

There is no vaccine for dengue that is currently approved in the United States, Dr. Paz-Bailey of the C.D.C. said. In any case, the vaccines that are available are not very effective, experts said, and there is a need for both more vaccine development and research into anti-viral drugs for dengue. The treatment now is to manage a patient’s pain and fluids.

The outbreak has led to emergency declarations in three Florida counties — Hillsborough, Pinellas and Pasco — and statewide support from the C.D.C., including $800,000 in emergency funds for things like mosquito surveillance and control.

On Oct. 10, Gov. Ron DeSantis issued an executive order adding mosquito control to an executive order to combat flooding because of Hurricane Isaias.

The outbreak has unfolded as Florida voters on Tuesday approved a property tax cut that critics assert will cost local governments billions in revenue and drastically reduce public services, including mosquito control.

Still, Dr. Longini said, the situation is not as dire as in other countries. “The size of the outbreaks in Florida compared to what we’ve seen in southern Mexico is quite small,” he said. “We’re not seeing tens of thousands of cases, we’re seeing hundreds.”

Infection is expected to decline as the weather cools down. Mosquitoes don’t breed as well and are less infectious in cooler weather.

The best way to prevent dengue is to avoid mosquitoes by wearing long sleeves and pants, applying insect repellents that contain DEET, using screens on windows and doors, turning on air conditioning and emptying standing water where mosquitoes breed.

General spraying “looks good politically,” but can be evaded by mosquitoes, Dr. Longini said. “It takes a lot of individual effort,” he said.

There are four different dengue virus types, and a second infection with a different type can lead to more severe disease.

The one fatality so far, Gay Small, may have had a repeat infection that made her illness worse, her family says. The cause of death was “multi-system organ failure secondary to dengue,” her daughter said.

Ms. Small had not traveled in the weeks before contracting the virus, her daughter said. Her family finds it paradoxical that she had traveled all over the world, often by bike, only to be infected by dengue in her hometown.

Now, part of her legacy may be the battle against the insect that killed her.

“They’ve been spraying like crazy,” Anna Small said. “Every night, I hear the helicopters over my house.”

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