Intestinal worms are parasites in the intestines, primarily on the walls. Among the different types of intestinal worms, the most harmful are roundworm, whipworm, pinworm, and tapeworm. Worms are caused by contaminated food or water, mosquitoes, sexual contact and even via the nose and skin. Other causes include poor hygiene, exposure to places which have parasites, weak immune system and eating raw and undercooked meat.