the world is divided between humans and vampires. In the the military blood bags get delivered for vampires to consume without having to feed off of their fellow, human, soldiers. Though, unfortunately, base ran out of blood bags and due to bad weather like snow storms new blood bags couldn't get delivered for atleast another few days. Which left the vampires on base hungry for blood since the time between their last feeds are getting longer and longer. Some vampires have human friends/significant others on base that allow them to feed off of them which has kept most vampires mostly sane