Factual error: The idea that anyone survived living in Pripyat Hospital is absurd. The uniforms of the doomed firefighters were stored in the basement and are so radioactive that anyone straying downstairs for more than 45 seconds has about a week to live. The ground and upper floors are far less radioactive but prolonged exposure - in other words, living there - would be fatal. Nobody could survive on the grounds for more than two or three months without receiving a lethal dose of radioactivity. Incidentally, prolonged exposure to radioactivity doesn't turn you into a deranged zombie - it turns you into a corpse.
Plot hole: Natalie uses her cell phone to record the injured and bleeding Chris, but it never occurs to her to use the phone to call for help? And how is it possible that in 2012, she seems to be the only one with a cell phone? It is simply not possible that she could not get a signal - the Chernobyl Exclusion Zone is festooned with mobile telephone masts - precisely because of the inherent danger of being lost there.
Factual error: The whole basis of the movie is flawed. You could not pick a more inappropriate location for a babes-in-the-wood style zombie film. The Exclusion Zone around the Chernobyl reactor site is not a deserted wasteland as shown in this film - it is one of the most secure and well patrolled areas on earth. It is crawling with police, military and paramilitary patrols and if you did manage to get lost all you'd have to do is walk or drive five minutes in any direction and you'd run into someone who could help. If there really was a population of mutated zombies about the place they would have been located and dealt with years ago. You cannot just drive into Pripyat (the setting of this film). To get there you would have to pass through two checkpoints, and a private vehicle would be refused entry. Further, the Exclusion Zone is (believe it or not) a popular tourist spot and the single road through is an important transit route for heavy goods vehicles driving to and from Kiev. In other words, you are never far from someone who could help, probably by shooting the zombie with his AK47. And as for mobile telephone masts - the place is festooned with them and you can make a call any time of the night and day.