FAQ for
[Rec] (2007) More at IMDbPro »

The girl in the penthouse is Tristana Medeiros, who was seen in the newspaper clippings that Angela and Pablo discover while inside.

The former resident of the penthouse was an agent of the Vatican who was charged with researching and isolating a suspected virus believed to be the biological cause of what was described as 'demonic possession', which was later confirmed to exist in a young girl (Niña). The agent kidnapped and brought the girl to the penthouse to conduct his research and to possibly cure her; unfortunately, during the work the virus managed to mutate and become contagious. The agent abandoned his efforts to cure her after failing to engineer a vaccine and sealed her in the room before leaving the city.

How the virus spread from then on - and the origins of the boy in the attic, remain unclear.