The origins of parasitism in the protists.

The origins of parasitism among the protists are, like the group itself, polyphyletic. Probably the majority of present-day parasitic forms evolved from free-living ancestors which were ingested as part of the food of their hosts, though origins from ectoparasitic forms and via a phase of facultative parasitism are possibilities, particularly among the ciliated protozoa and (for ectoparasitism) the Kinetoplasta. Sporozoan parasites most probably developed via a stage which was ingested and became adapted to life in the host's gut. Further developments in parasitism involved deeper penetration into the host's tissues and the adoption of more than one host in the life cycle, thus avoiding entirely the potentially hazardous phase of existence outside the host.