Parasite is an organism, animal or plant which depends for its survival on the other organism. The organism on which the parasite depends in called the host and the relationship between the two is called parasitism. Sometimes, parasitism leads to a high degree of specialization in their host requirements that some need only a single host while others need two or more specific hosts to complete their life cycle. Therefore, a regular alternation of generation may occur in such cases.
Parasites having association with single host are known as monogenetic parasites and have a simple life cycle. There are others (hat have relation with two or more hosts and have complicated life cycle with alternation of generations. Such parasites are called digenetic or trigenetic parasites depending upon the number of hosts.