Worms undergo three stages during their life cycle: egg, larva, and adult. The cat’s digestive system is the perfect environment for these parasites to live and multiply. Worm eggs are swallowed by a small animal, such as a mouse or bird, and grow into larvae that invade the host’s intestinal wall and develop into adults. There are over 350 species of worms in cats, most of which can be found in North America. Like humans, cats have a natural immunity to some worm infections.