In the 1930s the geneticist J.B.S. Haldane offered an explanation of why the gene for sickle-shaped red blood cells, which can produce lethal anemia, persisted in tropical populations. He suggested that the mutation offered a trade-off: although the sickle cells raised the risk of death, they also made a person one tenth as likely to contract malaria—a boon in the mosquito-ridden tropics. His striking idea that an infectious disease can drive evolution can now be directly tested in the laboratory with complex creatures, as reported this summer by Spanish researchers in the Proceedings of the National Academy of Sciences USA.