Tiger mosquito.

Asian tiger mosquitos, like the one seen above, can carry dengue fever and have been seen as far north as Chicago

Photo by Roger De Marfé/iStockphoto/Thinkstock.

In the autumn of 1885, people in Austin, Texas, began to feel sick. One after another, they developed a chill and then a soaring fever. They vomited and broke out in rashes. Their most distinctive symptom was agonizing pain behind their eyes and in the bones of their arms and legs. And when the fever subsided, lack of appetite and deep exhaustion left them unable to work for weeks or months.

Austin had been founded only 46 years before, and it was still small, with just 22,000 people. By the time the epidemic was over, 16,000 of them had fallen ill. A local doctor who described the outbreak in the Journal of the American Medical Association the following year added: “I am informed that other cities … had as many cases in proportion to the population as did Austin.”

The illness that took out Texas that fall had already devastated Charleston, S.C., in 1828and Savannah, Ga., in 1850, and it would go on to sicken half the population of Galveston, Texas, in 1897; one-quarter of Monroe, La., in 1922; and one out of every nine people in Miami in 1934. It was dengue—a mosquito-borne virus popularly known as “breakbone fever” for the pain it caused. From the 1820s to the 1940s, it caused recurring epidemics roughly every 10 years.

World War II mosquito-eradication programs broke the chain of transmission between humans and insects, and by the time the war ended, dengue had retreated to the tropics and was no longer a problem in the United States.

That may be about to change. At the annual meeting of the American Society of Tropical Medicine and Hygiene last month, researchers from the University of Florida revealed thatdengue has reappeared in Key West, Fla. The virus they found was not a one-time visitor imported by a tourist or a stray mosquito; it has been on the island long enough to become a genetically distinct, local strain.

The Florida researchers didn’t want to talk about their presentation because they hope to get it published soon in a medical journal. But it turns out other tropical-disease experts have been watching dengue’s return to the United States for a while and wondering what it will mean.

“It really is just a matter of time until dengue re-establishes itself in certain areas here,” says Amesh Adalja of the Center for Biosecurity of the University of Pittsburgh Medical Center. “The U.S. has been lucky that it has escaped so far.”

Dengue is already a pandemic elsewhere. Among insect-borne diseases, malaria gets the headlines: It causes about 219 million cases per year and about 660,000 deaths. But dengue is right behind it, racking up potentially 100 million infections per year around the world and putting about 500,000 people in the hospital, most of them children. It causes fewer deaths—25,000 per year—than malaria, but its prolonged illness keeps people from working and depresses both personal incomes and gross national product.

Dengue is also becoming more common. Between World War II and about 1970, severe dengue epidemics were recorded in only nine tropical countries; now the disease occurs routinely in more than 100. The primary driver has been the growth of slums as people leave rural areas to search for work. When migrants settle at the fringes of a city, they are beyond the reach of its infrastructure—water lines, sewer systems, and trash disposal—and they cope by digging latrines, storing water in jugs and barrels, and consigning trash to open dumps. All of those strategies create small pools of stagnant liquid, exactly the kind of habitat that the main dengue-carrying mosquitoes prefer. (The pools can be very small, less than an ounce.)

Dengue infects only humans and other primates—there is no intermediate host that harbors it, such as birds for West Nile virus—and people are its main vehicle for moving around the globe. After a bite, the virus replicates in the blood for four to seven days; once the fever starts, there are at least two and up to 10 days when the victim can cause an infection in the next mosquito that bites him or her. In the two weeks between the initial bite and the end of the infectious stage, a traveler can unknowingly transport the virus from an area where it is common—a marketplace in Singapore, a river terrace in Thailand, a beach in the Caribbean—to somewhere it has never been before.

The Centers for Disease Control and Prevention estimates that more than 2,700 peoplebetween 1977 and 1995, and more than 360 between 2001 and 2004, had that experience: being bitten somewhere, coming home, and getting sick afterward. In most cases, the disease went no further, but sometimes it kept spreading. There was a 122-person outbreak in Hawaii in 2001, the first time the virus had been seen in the islands since 1944. There were 25 cases in Brownsville, Texas, in 2005 and 90 cases in Key West between 2009 and 2010. In the last case, the outbreak extended over the winter, when cooler temperatures should have knocked out local mosquitoes.