A creature that lives off another organism is a parasite. The parasite might not hurt the host, but it doesn't do anything to help it, either.
If you call someone a parasite, you are really hurling an insult! The Ancient Greeks used the word parasitos to describe someone who ate at your table but never invited you back. Some telling synonyms include leech," toady, sponge, and hanger-on." Nice, huh? Some crafty birds, such as the cowbird or cuckoo, are called social parasites, laying their eggs in other birds' nests and expecting those mamas to raise their young for them. Unfortunately, there are some people like that, too.