[Selective pressure in host-parasite systems]

Selective pressures in host-parasite systems are the result of a continuous conflict between the divergent interests of each partner, on the long run. Whereas the fitness (lifetime reproductive success) of parasites is usually increased by a higher frequency of encounters with susceptible hosts and a better survival rate after infection, the fitness of hosts is increased by opposite processes, avoidance of encounters with infective stages and destruction of the parasites. These selective processes, often referred to as coevolution or arms races are in agreement with the Red Queen hypothesis of Van Valen, which assumes indefinite adaptive changes in both partners, in order to set up counter-measures against the weapons of "the other". Arms races in host-parasite systems thus suggest a gradualistic evolution, but this does not contradict the present day ideas on the tempo changes in the course of evolution (punctuated equilibria). Numerous factors, either genetic (evolutionary lag...), environmental (nutritional status...) or cultural (prevention, vaccination, therapy...) influence the severity of infections at an individual scale. The "terrain", which is a component of the individual phenotype, is thus at the cross-roads of genes, environment and culture. Humans must count more on their intelligence than on natural selection to prevent and cure infectious and parasitic diseases.