Parasitic stages of protozoans may be intercellular or intracellular occupants of host tissues, or occupy space within the lumen of various organs. There is not a common life cycle or sequence of morphological stages to represent this group. However, most of the parasitic protozoans exhibit one of three broad patterns.
The first pattern, common among many Apicomplexa (the sporozoans), incorporates an alternation between asexual and sexual reproductive stages. Cycles of schizogony in host tissues increase the population asexually, but at some point individuals undergo the sexual process of gametogony to produce gametes. The gametes unite and divide in the asexual process of sporogony to produce sporozoites which can infect a new host. Some species complete their life cycle in a vertebrate host, and transmit from host to host via cysts, containing sporozoites, which are passed externally. Other species use two hosts, a vertebrate host in which schizogony and gametogony occur, and an invertebrate vector in which gametes unite in the lumen of the gut and sporogony takes place in the tissues. In the two host life cycles, the parasite has no free-living stage and usually transmits between hosts by a blood-feeding vector.
The second pattern, common among flagellates, involves asexual reproduction almost exclusively. Various morphological transformations may occur, but all reproduce by binary fission. Some species complete their life cycle in a vertebrate host, and transmit from host to host via cysts, containing trophozoites, which are passed externally. Other species use two hosts, a vertebrate host in which some morphological forms occur in the tissues, and an invertebrate vector in which other morphological forms reside in the tissues. In the two host life cycles, the parasite has no free-living stage and usually transmits between hosts by a blood-feeding vector.
The third pattern, common among amebas, also involves asexual reproduction almost exclusively. A single host is used, which harbors trophozoites that live in the lumen of the gut and multiply by binary fission. Trophozoites may be stimulated to encyst and within the cyst they undergo nuclear division. The cyst is ingested by another host and after liberation from the cyst the parasite undergoes cytokinesis to produce new trophozoites.