Round worms they are usually born with and come from mom. After worming them for that it usually isn't ever a problem again. Tape worms come from eating fleas, not all fleas have them but all it takes is eating the right one. Other worms come from smelling infected feces or, rarely, hookworms, from infected dirt. Ferel cats are the main source of hookworm from what I have been told. If any of this is mis-information I apologize. Any corrections are appreciated.