Parasite

The parasite is a supernatural, demonic entity in the form of a worm-shaped organism, generated inside Tristana Medeiros after her possession. The seed of the demonic outbreak, it is able to infect other people and directly control them from inside. The parasite is introduced in REC 2 and returns in REC 4: Apocalypse.

History
The parasite was seemingly created after the Medeiros girl was demonically possessed, after she was raped by a group of priests. It remained inside her as her possession progressed and she was taken by Father Alberta to the Barcelona apartment building. Tristana's possession became able to spread in the form of a virus, which plagued every of the quarantined building's occupants, forcing the Ángela Vidal to hide inside the upper apartment where the infected Tristana was hidden. The parasite found a chance to leave Medeiro's body and forcibly transferred itself into Ángela, controlling her afterwards. After Father Owen stepped in the building attempting to find a cure for the outbreak, "Ángela" (the parasite) finally killed Medeiros' corpse and ordered Owen to allow her out, to which he insistently declined. The demon inside her revealed itself and murdered him, mimicking his voice and telling the authorities that "only one woman" had survived the outbreak.

After a second group of GEO's entered to take Ángela out, the parasite found a new host, passing itself to Guzmán, one of her rescuers. "Guzmán" and Ángela were taken to a quarantined boat, where a new outbreak occurred and it remained inside Guzmán. The head doctor on board, Ricarte, still believed it was inside Ángela until she proved herself free of any infection by biting him on his hand. Ángela then realized it could be inside Guzmán, to which "he" eventually confirmed. As the parasite tried to get inside her once again, Nick managed to aid Ángela, and she killed him with a harpoon. After Ángela and Nick escaped the detonating ship, it exploded, killing the horde of infected soldiers. The parasite, however, was left unharmed.

In the water, it was swallowed by (and dives into) a fish, finding its newest host.

Abilities
The parasite is able to control humans by forcibly entering them, and to communicate through other infected beings after an exorcism rite. It is able to mimic human voices. It is suggested that the parasite can also provide a cure for its own infection.

Notes and trivia

 * The Parasite prop used in Apocalypse was featured in the REC-themed exposition REC; Una Saga Indomable.
 * There is always on-screen interference when the demon manifests.
 * The parasites way of reaching human beings and controlling them resembles the alien worms from Hidden.
 * The Parasite is, in the REC universe, how demons physically appear.
 * Tristana Medieros is the only person possesded by the parasite that could infect other people, so it's likely that the parasite will remain stuck in the fish unable to do anything.