Parasitism

Parasitism is a relationship in which one organism benefits at the expense of another organism. A parasitic relationship consists of two organisms: the host, which is the organism being harmed, and the parasite, which gains all the benefit. Some parasites are only a little harmful. Fleas on dogs are an example of a parasitic relationship in which the organism survives. Other parasitic relationships are more severe. Tapeworms, which can affect humans as well as other animals, absorb incoming nutrients instead of the host. If the tapeworm becomes strong enough, or if the host is infected with multiple tapeworms, the host many become very sick or die. Brainworms can eat animals' brains, which can also lead to death.




Source
http://knol.google.com/k/klaus-rohde/parasitism-an-introduction-to/xk923bc3gp4/51#