Some puppies and kittens are born with intestinal parasites, which are passed from mother to pup during pregnancy. They can also be infected through their mother’s milk while nursing. Also while nursing, if they ingest a adult flea it will develop into a tapeworm in their system. After ingestion, you will likely start seeing the tapeworms in the stool approximately 3-5 weeks later.
Puppies and kittens should both be dewormed at least twice. Once at 2 weeks of age and another at 4-6 weeks of age.