Roundworm is a common parasite in puppies and kittens. The larvae is transferred to the developing puppy or kitten in the mother's uterus or during nursing. Pets can become infected by eating the eggs from the adult worm which are passed in the stool. It is important to have a stool sample checked at the hospital once or twice when you first get a new puppy or kitten and annually after that. Roundworm can be zoonotic - transferred to humans if the eggs are ingested.