Parasite and parasite are redirects to this article. For the meaning of the terms in the interpersonal sphere, see social parasite.

Parasitism (also called parasitism) refers to the phenomenon within the animal and plant world that an organism or virus (parasite) abuses a usually considerably larger organism of another species as a host. The host serves as the parasite's food supply (usually in the form of blood or other bodily secretions) and in some cases also as a long-term habitat. In the process, the host is damaged by the parasite, but usually remains alive. The consequences of parasitic infestation range from massive physical discomfort to severe infectious diseases and, in extreme cases, death, in addition to the sting or bite injury. The best-known and most widespread parasites of humans are, among others, human lice, mosquitoes and various species of ticks and worms.

Parasites whose parasitism usually leads to death are called parasitoids or predatory parasites, for example ichneumon flies. Parasitism is a form of interspecific interaction. If, on the other hand, no harm is done to the host, this is called probiosis, for example in commensalism.

In a broader sense, parasitism can be understood as an increase in the fitness of the parasite, sometimes associated with a decrease in the fitness of the host.