A form of Body Horror common in alien invasion plots. Aliens, rather than invading in their own form, insert themselves into (usually unwilling or unaware) humans, whereupon they completely take over the host’s body, suppress their will, and generally make them not themselves. They generally do this because their natural form is some kind of grub or other not-very-formidable state.
They may have limited or total access to the host’s memory, but can generally fool casual observers. A possessed host typically gains increased strength, and sometimes additional wacky powers. They may also be able to affect a Voice of Evil or glowing eyes, to let the audience know what’s up.