In recent decades, the disease has spread dramatically, from the nine countries that experienced epidemics before 1970 to more than 100 today, on every continent but Antarctica. It made its way to France and Croatia in 2010, and caused an outbreak of 2000 cases on Portugal's Madeira Islands in 2012. Florida, Texas, and Hawaii have reported dozens of cases in the past few years, almost all imported by travelers returning from the Caribbean or South America. But as Walter Tabachnic, director of the Florida Medical Entomological Laboratory in Vero Beach, recently told TIME magazine, "Sooner or later, our mosquitoes will pick it up and transmit it to us. That is the imminent threat."

Dengue fever is spread primarily by the aedes aegypti mosquito, and also by aedes albopictus, more commonly known as the asian tiger mosquito. Aedes aegypti is the culprit in nearly every dengue fever epidemic, while aedes albopictus gets credit for most outbreaks of chikungunya, an illness that produces symptoms similar to dengue, but is rarely deadly. Both mosquitoes are adept at transmitting these vector-borne illnesses to humans, as well as yellow fever and encephalitis. Both also thrive in urban environments, particularly indoors. Their eggs can hatch in the smallest bodies of water, including the pools of liquid that collect around shower and sink drains. Only the females bite, but they do so during the day, which means bed nets offer no protection. There is no vaccine for dengue, and no cure. Epidemics overwhelm local hospitals and crash local economies.

In most places where dengue fever and chikungunya are endemic, pesticides are a vital element of local strategies to prevent outbreaks, alongside public education campaigns encouraging people to rid their yards and homes of standing water. Local mosquito-control workers drive trucks down city streets, filling the air with toxic chemicals.

Besides being an environmental and human health hazard, fogging campaigns are not particularly effective, explains Haydn Parry. Parry is the CEO of Oxitec, a U.K.-based company associated with Oxford University that believes it has developed an alternative to pesticides: genetically modified (GM) mosquitoes. Oxitec’s mosquito carries a lethal gene that it then passes along to its offspring. The modified males are bred in a laboratory, then released into the wild, where they mate with local females, who lay eggs that will die before reaching adulthood.

"With insecticides, you have to take the product to the insect, which means you have to take your fog or spray into people's houses, and people don't like that. If a fogger machine is coming down the street, people close their windows and doors,” Parry says. “You know that 50 percent or more of the places mosquitoes are breeding are in people's houses, and you just can't get at them. The beauty of our little mosquito is that they don't have to ask permission to go on to someone's property. They are biologically programmed to find the females."