Despite the best care, your pet may be host to a variety of parasite pests during his or her lifetime. A veterinarian should catch any serious infestation during your pet's annual checkup, but knowing something about the pesky critters will help you keep them at bay. Parasites are creatures that depend on other creatures (including your pet) for their existence. Those that make their homes inside his or her body are called internal parasites. Those that live outside his or her body — fleas, ticks, mites and ringworm (which is actually a fungus) -- are called external parasites.