Medeiros

Story
The horrific monster was originally a Portuguese girl named Tristana Medeiros, who lived in a convent and used to work as a cooker. One night, while praying in her bedroom, Tristana was sexually assualted by a group of priests, found apparently dead in the following morning by a nun. Tristana woke up during her vigil, reacting violently to the priest's religious prayers and being tied with ropes on her bed by the nuns. The girl starred at the inverted crucifix above one last time and closed her eyes, demonstrating first signs of demonic possession right after opening them. Still tied, the demon inside Tristana killed the nuns by screaming loudly and breaking the glass windows on them. It then killed the priest by hitting him on the head with the crucifix. Horrified, the same nun who found Tristana after the rape night manages to calm her down, injecting a tranquilizer in her neck. A possessed Tristana is kept controlled with the injections and tied to a crucifix, until the Vatican priests arrive, one of them being Father Alberta. The case made ​​headlines and, some time later, Tristana was reported missing.

☀Somehow, Albelda moved Medeiros to an apartment building in central Barcelona, where the demon was chained in the penthouse apartment and sealed behind a door which would only open in complete darkness. Besides being a religious man, Albelda carried out scientific experiments with Medeiros's blood, using rats and local children as guinea pigs. The tests revealed to him that the root of the possession was a particular enzyme, which had now mutated and become an infectious disease, spread via saliva and bodily fluids. Albelda concluded that he needed to kill the possessed girl to avoid a possible contagion. Albelda poisoned Medeiros's food, composed of human flesh, and offered the meal to her. When the creature seemingly fainted, Albelda unlocked the chains and it suddenly awoke, chasing him throughout the apartment. Albelda climbed into the air ducts, where the other infected children were trapped, and locked himself there to die. Medeiros became a blind and emaciated figure, supernaturally locked in the darkness, until she found a way out when Ángela and Pablo entered the penthouse.

Father Albelda used rats to test the infection, and as the animals managed to escape, the virus made its way out of the attic. Ángela Vidal was a reporter that followed a group of firemen with her cameraman Pablo to the apartment building, where they ended up quarantined alongside the residents. After the demonic virus infected everyone except Ángela and Pablo, the two survivors were corralled to the apartment in the attic. In the dark room, they found newspapers reporting Medeiros's case and Albelda's records about his experiments. As there was no light, they were only able to see Medeiros through the infrared, and she was then able to escape and attack them. The demon grabbed Ángela and inserted a worm-like organism into the woman's mouth, possessing her body and gaining the ability to leave the darkness, and consequently, the attic.

Later that night, some floors below, she met a priest from the Vatican, Father Owen, who entered the building with a GEO team, seeking Medeiros's blood in order to create an antidote. The demon pretended to be Ángela, and followed the team to the attic where the demonic being somehow still lurked. The creature was then shot in the head by Ángela, and the demon completely assumed her body, ready to leave the place. As Owen didn't want to allow anybody to exit the building, Medeiros killed him and faked his voice on the radio, stating that only one woman had survived the outbreak.]The way the infection ended was when all of the infected victims in Barcelona and in the ship had burned, and when the infected in the wedding venue in the third movie as well as the infected in the zoo in the rec comics were all quarantined. The demon that caused the infection then gets destroyed by being devoured by a fish. And at this point, Ángela was already free from the possesion.

The virus
The Historias Ineditas comic book, which is mostly a prequel to the film series, suggests that while in the attic, Medeiros was able to remember her life as a "pure and pretty" girl, and that she lived in sadness due to her fate of being possessed by something evil. The second film states that the demon inside her can control the infected people and communicate through them, and the third film features Medeiros as the infected's reflection in the mirror, intensifying the idea that she is the seed of the outbreak.

Despite its supernatural origins, the demonic virus is described as working like a common virus in human blood. The time of symptom onset varies between each blood type, with some people immediately becoming blood-thirsty creatures, and others taking some time, languishing before the virus takes effect. The possessed people can be stalled by religious forces such as the recitation of scripture, generally evoked by a priest. The Medeiros demon manifests itself when religiously forced, in an exorcism-like ritual. They also manifest the eerie ability of mimicking the voices of others in order to trick and attack their victims.

Similar to the "infestation" (a supposed form of demonic possession), it also may affect animals. Apart from Jennifer's dog Max, the Historias Ineditas comic book explores an outbreak in a zoo, from where its infected animals (including the corpse of Copito de Nieve, the albino gorilla) then escape, on the same night as the first and second films' events.

According to the third film, the outbreak is actually the beginning (genesis) of freedom for the demons that managed to escape the darkness they were trapped in by God. In keeping with this, The Medeiros creature in the first two films seems to exist in a slightly different plane of reality than everyone else: It can only be seen (and can only attack and infect others) when there is total darkness. As well as the creature being revealed when the night vision of a camera is turned on, the geography of the apartment where it was confined changes in darkness as well: When a member of the GEO unit is pulled into a bath full of water in the apartment and the light is then turned on, the bath has been replaced by a more traditional sink unit and the man has vanished. The way the infection ended was when all of the infected victims in Barcelona and in the ship had burned, and when the infected in the wedding venue in the third movie as well as the infected in the zoo in the rec comics were all quarantined. The demon that caused the infection then gets destroyed by being devoured by a fish.