Parasites

Parasites, particularly intestinal worms such as hookworms, tapeworms and roundworms, can be transmitted in a dog's saliva or feces. Some parasites have fleas as intermediate hosts: the worm egg must be consumed by a flea to hatch, then the infected flea must be ingested (usually by the dog while grooming itself, but occasionally by a human through various means) for the adult worm to establish itself in the intestines. The worm's eggs then pass through the intestines and adhere to the nether regions of the dog, and the cycle begins again.