Parasite

Parasites are life-forms which require the assistance of other life-forms to reproduce, feed or survive - whether the host is willing or not, due to the idea of another living creature residing within oneself being a primal fear of many (as well as a deep instinctual fear of parasites and the devastating effect they can have upon population) these are a common antagonist in many forms of fiction, especially in the genre known as body horror (which specializes in the destruction or alteration of the body).

Parasites can come in a variety of forms but the most common subtypes are:

• Parasitoids: Some parasites deliberately kill their hosts, such as insects that lay eggs within hosts and have them consume from the inside out - the most iconic of these types of parasites are the Xenomorphs and The Thing.
• Symbiotics: A parasite that actually improves the host and while dependent on the host to survive does not weaken or kill the being, in fact it often bonds strongly and the host may even enjoy its company - a classic example is Venom and Carnage.
• False Immortals: Similar to vampires but take youth or life force rather than lifeblood - they will wither and die without it and are thus dependent on it, much like a drug. An example is Lord Voldemort.