Over half of all living species of plants and animals are parasitic, which by definition involves intimate association with and unfavourable impact on hosts (Price, 1980). This paper will only consider parasites whose ‘unfavourable impact’ adversely affects the birth and/or mortality rates of their hosts (Anderson, 1978). Most organisms are potential hosts and must deal with the problem of parasitism. The probability of parasitic infection of a host is influenced by both environmental and genetic factors. Traditionally it was assumed that a host was either resistant or susceptible to a particular parasite and therefore the interaction between a parasite and potential host had only two possible outcomes: either the resistant host rebuffed the parasitic attack and remained uninfected or the parasite successfully invaded and significantly reduced the reproductive success of the susceptible host. This approach, however, ignored the intraspecific genetic variation present within both host and parasite populations (Wakelin, 1978). Since the outcome is determined by the interaction of a finite set of host genes and parasite genes, genetic variation in host susceptibility and parasite infectivity (Richards, 1976; Wakelin, 1978) suggests that more than two outcomes are possible. Variation in host and parasite genomes does not begin and end at the susceptibility/infectivity loci. Other genes may also influence the outcome of host–parasite interactions by altering the life-history patterns of hosts and parasites, and lead to a variety of outcomes.
Host-pathogen coevolution increases genetic variation in susceptibility to infection