Dogs often have parasitic worms inside them. These can include a tiny tapeworm called Echinococcus. The larva forms a cyst the size of an orange inside the host animal. The worm's eggs are passed out in dog faeces. A dog that has licked itself clean may have eggs on its tongue. If it licks the face or hand of a human, they may be infected. A huge cyst can form in their body, or most deadly of all, in the brain.