The horrific monster was originally a Portuguese girl named Tristana Medeiros, who lived in a convent and used to work as a cook. One night, while praying in her bedroom, Tristana was sexually assaulted by a group of priests and found by a nun in the morning, appearing to be dead. 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 stared at the inverted crucifix above her one last time and then closed her eyes, demonstrating the first signs of demonic possession immediately after opening them. Still tied, the demon inside Tristana killed the nuns by screaming loudly and breaking the glass windows onto 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 manages to calm her down by injecting a tranquilizer into 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, Alberta took Medeiros to a pension building in Barcelona, where the demon was chained in the attic. Besides being a religious man, Alberta carried out scientific experiments with Medeiros's blood, using rats and nearby children as guinea pigs. The tests revealed to him that the possession worked as an infectious disease, in a way similar to influenza. Alberta then concluded that he needed to kill the possessed girl, who had become a horrendous creature, to avoid a possible contagion. Alberta poisoned Medeiros's food, composed of human parts, and offered the meal to her. When the creature seemingly fainted, Alberta unlocked the chains and Medeiros woke up, leaving her closet to chase him throughout the apartment. Alberta climbed to the air ducts, where the other infected children were trapped, and locked himself there to die. Medeiros began living in the apartment, supernaturally locked in the darkness, until she found a way out when a young woman named Ángela Vidal entered the room.
REC Part 1 and 2Father Alberta 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 pension, 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 the news reporting Medeiros's case and Alberta'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 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 a violent Ángela, and the demon completely assumed her body, ready to leave the place. As Owen didn't seem 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.
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. Zombified people can be stalled under religious forces, generally evoked by a priest. The Medeiros demon manifests itself when religiously forced, in an exorcism-like ritual. They also manifest the strange ability of mimicking the voices of others in order to attack their preys.
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.