This story picks up, approximately, where [REC] (2007) ends.
A special task force enters the quarantined apartment building in search of the source of the infection. With them is a priest, a representative of the Catholic Church, who will go to great lengths to gain the sample they need to develop a cure.
Spoilers Ahead!
What we find out, is that it’s not a “rage” virus, like in 28 Days.. an actual demonic possession is being transmitted from one victim to another. In their experiments, we see that the infected are greatly affected by holy means, such as crucifixes and holy water.
I see why this franchise has become popular in the horror community, for a low-budget production, [REC] is very good, and even better with the subtitles, instead of English dubbing.