The interaction of the behavior and ecology of hosts with the biology of pathogens as relating to the impact of diseases on populations. Although human diseases have been studied from an ecological perspective for centuries, diseases in plant and animal populations have only recently received much attention. The reason that ecologists have generally overlooked diseases is that diseases tend to be “invisible” compared to predators or competitors. The toll that diseases can take on all organisms (even bacteria are attacked by diseases) is enormous, and the ecology of diseases is one of the most vigorous areas of research in modern biology.