Parasitism and evolution: opposing versus balancing strategies

Parasitism refers to a particular symbiosis (deBary 1879) of organisms that live at the cost of their hosts. Virulent microbes (viruses and bacteria) proliferate unlimited causing toxic infections that end either by the host's death or its protective immunity. By this opposing survival strategy, via the population density, i.e. the contact rate, finally results a sequence of epidemic and endemic periods. The host population is kept at a reasonable level to resources, but host and pathogenic agent are wasted in the evolutionary arms race. Sparing such losses, eukaryotic proto- and metazoan parasites induce in natural hosts a delayed infection course by self-controlled propagation and avoid protective immunity. Hosts are ?immunized but not immune? and parasites propagate simultaneously with them. Metazoic and some protozoic parasitoses end with the parasite's natural death, pathogenic effects remain irrelevant. Nevertheless, the host's fitness becomes reduced by multiple re-infections and/or by increasing parasite loads. This balancing survival strategy controls host populations at lower costs. A network of parasitoses offers considerable selection advantages and explains the polyphyletic origin of eukaryotic parasites. Parasitism and other kinds of symbioses contribute to the stability of ecosystems and to stasis in long-term evolution but are invisible in the fossil record.