[REC] 2 (2009)
This takes the horror of [REC] and explains everything, for better or for worse. I vote worse, because the explanation doesn’t make sense logically. It’s demonic possession, but spread via the blood. I guess that’s a unique take, but somehow I think someone experimenting with a bioweapon in their apartment would be more terrifying.
They wrote themselves into a corner a little in the last one. The way they got out of it is pretty clever.