Parasite

A parasite is a living thing that makes its home on or inside another organism, the host, and draws its food from that host, to the host's cost. The relationship, called parasitism, benefits the parasite and harms the host, although a well-adapted parasite often weakens its host rather than killing it quickly. Parasites living inside the body are endoparasites; those on the outside, such as ticks, fleas, lice and mites, are ectoparasites.

In human medicine the word usually covers three groups. Protozoa are single-celled organisms that can multiply inside the body, so a few can grow into a heavy infection. Helminths are parasitic worms, mostly visible to the eye as adults, which do not multiply as adults inside a person. Ectoparasites can cause disease directly, but they matter most as carriers of other pathogens. Many parasites have complex life cycles with more than one host: a definitive host, in which the adult parasite reproduces sexually, and one or more intermediate hosts.

Disease-causing bacteria and viruses also live at a host's expense, but by convention parasitology focuses on protozoa, worms and arthropods. Malaria, caused by Plasmodium, kills more people than any other parasitic disease.