A parasite is an organism that lives off, and generally within, a host body. This can include your body, or the body of an animal, like your pet. Parasites live off of the "life" of another body, feeding on the nutrients, cells, and organs, of the host. They can reproduce by the thousands, depositing thousands of eggs, or simply replicating by cell division, within the host's tissues and cells. Estimates are that up to 85% of the human population is infected with parasites, and up to 50% of people in the United States are infected.

Parasites are often "mobile," moving to feed off of one area of the body and then to another. They can eat the host's cells directly, or drain the best of the nutrients directly from the host's tissues. (Remember, if you are inhabited by parasites, we are not talking about some other "host," but we are talking about YOU.) After eating, the parasite excretes its fecal wastes throughout the body's host