Introduction

This paper aims to give an overview of the dynamics and genetics of interactions between populations of viruses and their hosts. I emphasize that ‘successful’ parasites are those individuals who leave the most offspring, and that this does not necessarily mean evolving toward being harmless to the host (even though this may be the best for the viral population as a whole); many evolutionary trajectories are possible, depending on the constraints and trade-offs among viral virulence, transmissibility, and the costs of host resistance. These general ideas are illustrated by the particular cases of myxoma virus and Australian rabbits, and of HIV-1 and human hosts (where the role of ‘escape mutants’ of the virus, and of a possible ‘diversity threshold’ beyond which the immune system can no longer control the virus, are emphasized).

Viruses as ‘microparasites’

Viruses and other infectious agents that afflict humans and other animals are conventionally classified along taxonomic lines. In discussing the ecology or evolution of host/parasite associations, however, it is often more useful to make distinctions on the basis of the population biology of the interaction.

Microparasites (sensu Anderson & May, 1979) are those which have direct reproduction, usually at very high rates, within the host. They characteristically have small sizes and short generation times; the duration of infection is typically very short relative to the expected lifespan of the host, and hosts that recover from infection usually acquire immunity against re-infection for some time, often for life. The result is that most microparasitic infections are of a transient nature in individual hosts.