FITNESS OF PARASITES - PATHOLOGY AND SELECTION

Parasites improve their fitness as a result of the selection of traits which determine their relationships with their hosts, Some of these r elationships are examined briefly. There is a cost of virulence for pa rasites, paralleling the cost of resistance for hosts, which implies t hat the good health of the host can be a component of parasite fitness ; conversely, some transmission modes imply that the host be markedly weakened by the parasite. Pathogenicity can be influenced by character s such as a transmission of the parasite from parents to offspring, or the demographic characteristics of the host populations. Important co mponents of parasite fitness are: the complexity of the life-cycle; th e degree of specialization for a more or less open host range; the con spicuousness or discretion of the infective and parasitic stages, Howe ver, the best possible adaptation to a particular host is not always s elected: when a parasite exploits several host species, the gene flows between parasites which have developed in different hosts may be resp onsible for ''maladaptation''. This may be important for an understand ing of the pathogenicity of certain human parasitic diseases. Copyrigh t (C) 1997 Australian Society for Parasitology. Published by Elsevier Science Ltd.