Host density and the evolution of parasite virulence.

Social and cultural habits of human populations affect the biological evolution of the agents of infectious diseases. Measles and similar diseases have evolved in the Old World and cannot have existed in their present form before the rise of the great river valley civilizations. It is suggested that increased virulence of measles in white and indigenous communities in America 1500-1800 may be due to a rare strain of the virus, which was selected during transfer from Europe. The release of viruses for biological pest control has provided new material for the study of the co-evolution of host-parasite systems, which has upset the dogma "evolution tends to avirulence". It is pointed out that this issue is closely related to the group selection debate among ethologists, i.e. to the problem: how can group selection overcome individual selection? A model is proposed in which differential growth of two strains of a parasite within the host and their transmission to new hosts is considered. It is supposed that transmission stages excreted by infectious hosts enter a common pool where they are mixed before infecting new hosts. Under these conditions, selection of the slower strain is possible only if the mean size of parasite inoculum is very small, i.e. if the density of transmission stages in the environment is low. The impact of this result on host pathology depends on the relation between virulence and transmission efficiency of the parasite.