Answer:
A parasite. There are 2 kinds of parasites. The first does not effect it's host but benefits by having a plae to live/survive. The second benefits both the host and the parasitic organism (fun fact: The 'black suit' as seen in Spider-Man 3 is techniquelly a parasite belonging to the second catagory: it lives off of anger and hatred and other negative emotions while at the same time it enhances ones phsyial abilities).