“Parasitism is the association of two organisms of different species in which one, termed as parasite, totally or partially depends upon or lives within the larger animal, the host, from which it obtains nourishment and shelter without any compensation for the host”. In this association the parasite is benefited and the host is harmed. Parasites occur in all the classes of protozoa. It has been estimated that about 1/5th of all protozoans are parasites. The association between the parasite and the host entails certain changes or adaptations in the parasite so that it can survive and derive maximum benefit from the host. Similarly, the presence of the parasite in the host body is bound to affect the host to a greater or a lesser degree. This kind of relationship is called as Host-parasite relationship.