There are important interactions between disease and organic evolution, between disease and cultural evolution, and between all three. Social behaviour influences disease and is influenced by it. Disease and disease mortality are woven into the complex of behavioural and physiological reactions to the stresses of overpopulation, which act to reduce population size. These principles are illustrated with reference to a number of diseases, including vitamin D imbalance, phenylketonuria, lactose intolerance, malaria, sickle cell anaemia, favism, plague, yellow fever, syphilis, ergot poisoning, kuru, and the sweating sickness.