Many protozoa have symbiotic lifestyles. Symbiosis is an intimate association between two organisms. For many protozoa, these interactions involve a form of symbiosis called parasitism, in which one organism lives in or on a second organism, called a host. The host is harmed but usually survives, at least long enough for the parasite to complete one or more life cycles.
The relationships between a parasite and its host(s) are often complex. Some parasites have life histories involving multiple hosts. The de?nitive host harbors the sexual stages of the parasite.
The sexual stages may produce offspring that enter another host, called an intermediate host, where they reproduce asexually.
Some life cycles require more than one intermediate host and more than one immature stage. For the life cycle to be complete, the ?nal, asexual stage must have access to a de?nitive host.
Other kinds of symbiosis involve relationships that do not harm the host. Commensalism is symbiotic relationship in which one member of the relationship bene?ts, and the second member is neither bene?ted nor harmed. Mutualism is a symbiotic relationship in which both species bene?t.