Resulting from a Royal Society discussion meeting, this volume presents a short review of the topic of parasite-host co-evolution. Current thinking on evolution in parasites, viruses and other pathogens is discussed.

1 Selection by parasites for clonal diversity and mixed mating.- 2 Recognition and polymorphism in host-parasite systems.- 3 Viral pathogens and the advantage of sex in the perennial grass Anthoxanthunt odoratum.- 4 Mate choice and maternal selection for specific parasite resistances before, during and after fertilization.- 5 Infection and colony variability in social insects.- 6 Infectious diseases, reproductive effort and the cost of reproduction in birds.- 7 Evolution of mutation rate and virulence among human retroviruses.- 8 Gene-for-gene recognition in plant-pathogen interactions.- 9 MHC polymorphism and parasites.- 10 Natural selection at the class II major histocompatability complex loci of mammals.- 11 The role of infectious disease, inbreeding and mating preferences in maintaining MHC genetic diversity: an experimental test.- 12 Human leukocyte antigens and natural selection by malaria.