What is a Parasite?

I am often asked “What is a parasite?” “Are bacteria parasites?” “Are viruses parasites?”

Few parasites have a worse reputation
   than the scabies mite. Image by
    Kalumet CC BY-SA 3.0
     (background edited)

A working definition of a parasite is “an organism that lives on or in another organism and gets what it needs for survival and reproduction from that host organism.” In Foundations of Parasitology (6^th ed., McGraw Hill, 2000) Roberts and Janovy define a parasite as an organism that “either harms its host or in some sense lives at the expense of the host” (p. 6).  Robert S. Desowitz wrote that parasites “have evolved from free-living forms who through opportunity, mutation, and selection have come to live in or on another organism” (The Malaria Capers, Norton, 1991, p. 94).

By these definitions, bacteria and fungi living in us, and on us, are clearly parasites. Whether viruses are parasites or not depends on whether one believes that viruses are alive – but that’s a discussion for another time. These definitions provide an objective, biological view of what a parasite is. From a medical point of view, parasites include protozoa, worms, and things like lice, fleas, and bedbugs. They do not include bacteria, fungi, and viruses. It’s a division of labour: microbiology gets the bacteria, mycology gets the fungi, virology gets viruses, and parasitology gets everything else that wriggles, jumps, crawls, swims, or squelches along.

And from a medical point of view, parasites are bad: they harm the host and are no earthly good whatsoever. Usually they do seem bad. We are only just beginning to realize that human parasites may have some important redeeming qualities – and any definition of parasite is a long way from acknowledging that.

Parasite: the word itself denotes something bad.