Juazeiro, a city in northeast Brazil, was a great place to try them out. After it was wiped out for 20 years, dengue has been on the rise in Brazil, with an estimated 16 million new cases every year. Many of the mosquitoes that carry the disease are also resistant to pesticides, which meant that Brazilians were left with few options to decrease dengue’s prevalence. The neighborhood in which the researchers tested the modified mosquitoes was a low-income area with high rates of dengue infection, according to local public health officials. Over a one-year period, the researchers released the modified males into the local environment and monitored the resulting eggs, looking for a characteristic fluorescent marker engineered into the males’ genome. In the course of that year, the number of disease-carrying mosquitoes decreased by 95 percent as compared to a control group in a neighborhood next door.