Survival of a parasite requires that the parasitic species must be able to spread from one susceptible host to another susceptible host. Parasites that spread directly from definitive host to definitive host have a direct life cycle . Some parasites have adapted to life by producing transmission stages that live, develop, and persist in other organisms in the surrounding community. The parasite does not undergo sexual reproduction in these organisms, but required development , or transformation from one life cycle stage to another, occurs. Organisms that are required for the development of immature or asexual stages of parasites are known as intermediate hosts .
Asexual reproduction is possible with certain parasites in their respective intermediate hosts.
Intermediate hosts become infected with immature or asexual stages of parasites by ingesting transmission stages, such as eggs, larvae, and cysts, produced by parasites in the definitive host, by ingestion of post-transmission stages inhabiting other intermediate hosts, etc . Definitive hosts acquire parasite transmission stages by eating various types of infected intermediate hosts, or by being fed on by hematophagous arthropod intermediate hosts. Parasites do not undergo substantial change in form inside a paratenic host , although some parasites may increase in size in a paratenic host. In the known parasite-paratenic host relationships that have been studied, the parasitic stages can persist for long periods of time in the paratenic host. Parasites may be transmitted between intermediate and paratenic hosts, and likewise transmission may occur between multiple paratenic hosts.