As for me I like horror movies which action is closely linked to happen in real life.
The best I could think of is actually a deadly viruses, dangerous genes experiments, mutating zombies and all that biohazardous threat to mankind has always freaked me out when I start to think about it, you know like in the Resident Evil 1 or Quarantine.
Who knows maybe there are already top secret military facilities somewhere, where these kind of viruses has been already developed
