Serratia usually lives in the midgut of the mosquito, but scientists realized that some strains also colonize mosquito ovaries, and a gland that males use while mating. Because of the bacteria's preferred locales, the genetically modified bug can spread during sex—and to the surface of resulting mosquito eggs. That means not just the infected mosquito's offspring pick up the bacteria: all the larvae growing in the same water receive the genetically modified germs too. "There are very few bacteria that are able to spread in this way," says Marcelo Jacobs-Lorena, a microbiology researcher at Johns Hopkins. The modified Serratia creates a compound that makes it hard for the malaria parasite to continue to multiply and thrive, but does not harm the mosquitoes that would usually carry that parasite and pass it on to us.