Intestinal parasite: A parasite (an organism that lives in or on and takes its nourishment from another organism) in the intestinal tract. Intestinal parasites include both helminths and protozoa. Helminths are worms such as tapeworms, pinworms, and roundworms. All of these worms can live, but typically not reproduce, inside the human intestine. In contrast to worms, which are composed of many cells, protozoa are single-celled organisms that can multiply inside the body. Examples of protozoa that can live in the intestinal tract are Giardia and Cryptosporidium.