Symbiont

A parasite is any creature that invades a host and thrives at its expense. Once it has made itself at home, it could cause any number of effects to its host. Possibilities include injecting a venom, releasing a disease, forcing internal bleeding, draining its host of some vital resource (e.g. oxygen, food, water, MP), laying eggs, or even taking bodily control of the host. Some parasites actually cause beneficial effects in place of or in addition to detrimental ones, such as heightened strength, speed, durability, influence, or immunity.