This week, Hillsborough County in Florida, which includes Tampa, declared a state of emergency linked to a growing number of dengue cases. So far this year, around 485 cases have been reported in the state, according to the CDC. The disease, which is transmitted by mosquitoes, can cause serious flu-like illness, and one person in Florida has died in the outbreak so far this year, the state’s department of health told media outlets in September.
Seasonal outbreaks of the disease, which affects up to 400 million people worldwide annually, are not unusual in Florida, and they’re often linked to travelers. But this time is a little different, says Michael Johansson, an epidemiologist at Northeastern University. Dengue, sometimes called dengue fever, is caused by a collection of viruses carried by mosquitoes, primarily Aedes aegypti.
Dengue is endemic in a wide swath of the tropics, and usually, most Americans who have the infection acquire it outside of the U.S. during travel. Dengue symptoms can occur 5-7 days after being bitten and range from mild to severe. They may include painful body aches as well as fever and can last up to 10 days.
However, in parts of the U.S. where Aedes aegypti mosquitoes live, they can bite infected people and start to spread the disease locally. Usually that looks like just one or two cases, but sometimes there are small outbreaks.” In 2009, when two locally acquired infections were identified in Key West, public-health surveillance revealed that a substantial share of people in the town had markers in their blood showing a recent encounter with dengue, even if they hadn’t had symptoms. That suggests that in an outbreak, dengue is likely to be infecting more people than might immediately be clear from case counts.
One distinctive feature of dengue, however, is that the second time someone is infected is likely to be much worse than the first. Sometimes a second infection can progress to dengue hemorrhagic fever, which can involve bleeding and a drop in blood pressure, and is sometimes fatal. This pattern, where the second exposure is more dangerous than the first, has complicated attempts to vaccinate people against the disease.
In 2017, Sanofi Pasteur, the manufacturer of the dengue vaccine Dengvaxia, alerted the government of the Philippines that serious dengue infections might occur in people who had received the vaccine as their first exposure to the disease. No dengue vaccine is currently available in the U.S.; Dengvaxia was available starting in 2019 but in 2025 was discontinued by the manufacturer, which cited low demand. So far, dengue has not established a permanent foothold in the continental U.S., says Johansson.
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