An organism that lives on or in a different kind of organism (host) and feeds off the latter.

The parasite without its host usually dies. Because of its way of survival, the parasite undergoes drastic alterations (e.g.: disappearance of motor, sensory and other organs; enlargement of genital organs, creation of adhesive organs) resulting in irreversible specialization-mutation.

For the parasites, creating offspring acquires enormous dimensions as a counter measure to losses due to a lack of host. The host is always damaged, sometimes less, sometimes more, which might even lead to death. The parasitic populations facilitate in the spread of epidemics, and are one of the major factors in regulating the population of animals.