Parasite

Parasite: An organism that lives in or on and takes its nourishment from another organism. A parasite cannot live independently.

Parasitic diseases include infections by protozoa, helminths, and arthropods: Protozoa -- Malaria is caused by plasmodium, a protozoa, a single-cell organism that can only divide within its host organism. Helminths -- Schistosomiasis, another set of very important parasitic diseases, is caused by a helminth (a worm). Arthropods -- The arthropods include insects and arachnids (spiders, etc.), a number of which can act as vectors (carriers) of parasitic diseases.

The term "parasite" came from the Greek "parasitos" (para-, along side of + sitos, food) meaning "eating at the side of, as at the same table." The sense of the term later changed to that of a poor friend or relative who lived at the expense of another. Not until the 18th century did "parasite" come into English as a biologic term.

The study of parasites is parasitology.