One key issue in the ecology of infectious diseases is the extent to which biodiversity affects the occurrence of disease. There is theory and evidence for and against the notion that high biodiversity will reduce the impact of infectious diseases on host populations. A new paper provides observational and experimental evidence that biodiversity can have a substantial impact of disease occurrence and this is discussed in an excellent piece of science journalism in Qualia, a science blog from the American Association for the Advancement of Science: <http://membercentral.aaas.