Intestinal parasites are typically referred to as worms. Almost all puppies and kittens are born with intestinal parasites which may cause them to remain thin with a large pot belly appearance and persistent diarrhea. All intestinal parasites rob their host of nutrition and if left untreated they can lead to death. And they can be transmitted to humans.
Hookworms, like Roundworms, live in the small intestine. The hookworm is microscopic in size and attaches to the intestinal wall with hooks and then sucks blood. It causes pain, diarrhea, anemia and general poor health and it impedes normal growth. In severe infestations they can cause death.
Roundworms are the size of piece of spaghetti and live inside of the small intestine. They take nutrition from their host and in combination with Hookworms they can lead to dull hair coat, diarrhea, vomiting, a pot bellied appearance and death.
Whipworms live in the large intestine and cecum (similar to our appendix) of dogs and rarely cats. Whipworms cause intermittent pain and bloody diarrhea. By themselves they rarely cause death but they do cause weight loss and malnutrition.
Tapeworms live in the small intestine of both dogs and cats. They can become feet long if left untreated. Tapeworms shed segments which are motile and contain the eggs. They are frequently seen as grains of rice around the anus of infected animals. The eggs hatch and a flea eats the larvae and then a pet eats the flea. They do not seem to cause much harm to the pet.
Coccidia are not technically considered worms. They are a single celled protozoan organism that lives inside the cells of the small intestine. When they are ready to reproduce they rupture the cell they are living and pass eggs into the feces. When they rupture the cells they cause inflammation and pain and diarrhea which many times may be bloody. They are usually found in puppies and kittens and Coccidia of animals does not infect humans.