Parasitism.
Résumé
Parasitism is a pervasive phenomenon in nature involving, either as hosts or as parasites, virtually all species on Earth. By definition, parasites are costly for their hosts as they divert resources for their growth, reproduction, and survival with no rewards for the hosts. Parasitism is not restricted to a few taxonomic groups as a parasitic lifestyle has evolved in groups as diverse as viruses, bacteria, protozoa, invertebrate and vertebrate metazoan. This taxonomic diversity is, of course, also associated with a tremendous diversity of life cycles, host exploitation strategies, transmission modes, and virulence levels. Given the cost of parasitism, hosts are expected to evolve defense mechanisms aiming at limiting the negative effect of parasitism on their fitness. In agreement with this view, hosts have evolved a series of morphological, physiological, and behavioral adaptations to fight off parasitic attacks. On their side, parasites have responded to the selection pressures exerted by their hosts by evolving counter-adaptations to overcome host defenses. These cycles of host–parasite adaptations/counter-adaptations define the so-called coevolutionary process, one of the most prominent characteristics of host–parasite interactions.