Dogs pass the infectious tapeworm eggs in their stool, which can cause infection in people who accidentally swallow the eggs. This can happen quite easily if, for example, you pat your dog and then don't wash your hands before eating. There are 2 species of tapeworm which commonly affect humans, Dipylidium caninum and Echinococcus granulosus. If a human is infected by Dipylidium caninum, worms will develop in their digestive tract. Unless the worm burden is heavy, this rarely causes harmful disease. Echinococcus infection, however, is very serious. Infection of humans by Echinococcus tapeworm results in slow growing hydatid cysts to develop in different organs of the body. This can result in organ disease, cause embolisms, or fatal anaphylactic reactions. The cyst can be very difficult to treat.
To prevent tapeworm transmission to humans you should ensure you are regularly treating you dogs for worms. Adult dogs should be wormed every 3 months. Practice safe hygiene by picking up any stool in your yard, wash your hands after handling your animal, and avoid having dogs lick your face. If you suspect your animal has worms you should be especially conscious of children who are at greater risk of infection.