I haven't read the rest of the comments, but here are my ideas!
A: The infection could mutate and become air-borne or water-borne (If thats how it's spelled)
B: The infection could mutate, and the infected could become inteligent with sudden bursts of instinctual rage. Which makes one able to sneak into the compound farm thing thing.
C: The disease has created new living conditions, creating new beasts, whom are either immune to the disease or relies on the disease to survive. These beasts are either huge and killer, or small and fast with deadly results. The immune creatures aren't as agressive as those who relies on the disease to survive.
D: The compund simply runs out of resources.
E: A new type of disease is spreading, turning people into look-alikes, which will make their very own kind shoot them. Even though they aren't deadly or vicious.
F: An earthquake or another happening destroys the defences completely (The farms defences)
G: A helicopter lands on the farm, telling that a cure has been found, and that everything is going to be alright soon enough. As they laugh reassuringly, they get a radio call from their base, where the one calling is about to be eaten, as he shouts for help.
I'm sure I could come up with more, but I think thats plenty. I loved the game! :3
yumza18
You really should have the compound be breached and then the main character battle through the city and end up in nastya's outpost like in the online dead frontier