In the 1960s, the World Health Organization (WHO) began a campaign to eradicate' the mosquitoes that transmit the disease malaria. It was a noble goal, since malaria kills an estimated 3 million people each year in the world's tropical regions, predominantly southern Africa. WHO led an effort to spray the mosquitoes' habitat with a chemical pesticide—a poison used to kill insects—called DDT. Early results were promising, and the mosquito was eliminated from the edge of its native range. The effort soon faltered,' however, and the eradication plan was dropped. How could a tiny mosquito thwart' the best efforts of a large group of wellfunded scientists?