REC

Reporter Angela Vidal (Manuela Velasco), and her cameraman Pablo, are covering the night shift in one of Barcelona's local fire stations for the documentary television series While You're Sleeping. While they are recording, the firehouse receives a call about an old woman, Mrs. Izquierdo, who is trapped in her apartment and screaming. Ãngela and Pablo accompany two of the firefighters, Ãlex and Manu, to the apartment building, where two police officers are waiting. As they approach the old woman, she becomes aggressive and attacks one of the officers, biting his neck.
As they carry the injured officer downstairs, they find the building's residents gathered in the lobby. The police and military have sealed off the building and trapped them inside. As people begin to panic, Ãlex, who remained upstairs with the old woman, is thrown over the staircase railings and seriously injured. The old woman then kills a girl, and the remaining officer, Sergio, is forced to shoot her. Ãngela and Pablo begin interviewing the residents, including a sick little girl named Jennifer. Her mother Mari claims she has tonsillitis, and says her dog, Max, is at the vet because he is sick as well.
The injured are put in the building's textile warehouse. A health inspector in hazmat suit arrives and attempts to treat them. Suddenly, they become aggressive and start attacking other people. The residents flee and Guillem, an intern, is locked in the warehouse. The health inspector explains that they are infected with a virus similar to rabies, and the time in which the disease takes effect varies by blood type. He reveals the disease is traced back to a dog in the apartment building, and Ãngela realizes it's Max. When the residents confront Mari, Jennifer turns, bites her mother's face and flees upstairs.
Sergio handcuffs Mari to the stairs and proceeds upstairs with Manu and Pablo. They find Jennifer but she bites Sergio, who tells the others to leave him. Manu and Pablo find the remaining residents running upstairs as the infected in warehouse have broken down the door. Leaving the handcuffed Mari behind, they enter an empty apartment along with Ãngela, a resident called CÃ©sar, and the health inspector, who has been bitten. CÃ©sar mentions that there might be another way out through the basement, where there is a large drain that is connected to the sewers, but says the keys are in Guillem's apartment. The infected health inspector then bites CÃ©sar, forcing Ãngela, Manu and Pablo to escape and fight their way up to Guillem's apartment on the fifth floor.
Having found the key, Ãngela and Pablo leave the apartment and find Manu among the infected. The pair are chased upstairs, and take refuge in the penthouse. They discover a tape recorder which explains the virus' origin. The penthouse' owner was an agent of the Vatican. He was researching and isolating an enzyme believed to be the biological cause of demonic possession. He located a possessed young girl named Tristana Medeiros, kidnapped and brought her to the penthouse for research. During this time, the enzyme mutated and became viral. The agent, having no other options, sealed Tristana in the house, presumably to let her die of starvation.
Pablo reaches up with his camera to record around inside the attic. An infected child swipes at the camera and breaks its light, shrouding them in complete darkness. Pablo turns on the night vision and discovers a sealed door. Tristana, now a blind and horribly emaciated figure, emerges and searches the penthouse for food. Ãngela and Pablo try to escape, but Pablo is killed by Tristana and drops the camera. Ãngela then picks it up and looks through the screen. Seeing Tristana eating Pablo, she panics, trips and drops the camera. The camera continues to record as Ãngela is dragged into the darkness screaming.

Question: What causes the virus to be mild or severe?
Answer: Blood type