Scientists are breeding large numbers of mosquitoes, potentially billions, as part of efforts to fight disease. The use of genetically modified or sterilized mosquitoes, as well as those infected with Wolbachia bacteria, is being explored in various locations, including Brazil, California, Florida, Tanzania, and Tahiti. The goal of these initiatives appears to be controlling the spread of diseases such as malaria and dengue, although the specifics of the approaches and their potential outcomes are not clear.