Parasites that populate the gastro-intestinal tract. In humans, they are often spread by poor hygiene related to feces, contact with animals, or poorly cooked food containing parasites. The major groups of parasites include protozoans (organisms having only one cell) and parasitic worms (helminths). Of these, protozoans, including cryptosporidium, microsporidia, and isospora, are most common in HIV-infected persons. Each of these parasites can infect the digestive tract, and sometimes two or more can cause infection at the same time.