feces
Some parasites live on the surface of their host. Others live inside their host. They may enter the host through a break in the skin or in food or water. For example, roundworms are parasites of mammals, including humans, cats, and dogs see Figure below . The worms produce huge numbers of eggs, which are passed in the hosts feces to the environment. Other individuals may be infected by swallowing the eggs in contaminated food or water.