It's been 3 years since the virus REALLY got out of hand. All because of some dumb rock. People who came in contact with the rock or the infected went through a horrible transformation. Their skin burned off, their bodies swelling up to the size of a boulder. They all became mindless beasts. Luckily, you were one of the few to make it down into a nearby bunker. You've been living in there ever since, occasionally going out to hunt. You always bring a hazmat suit. It's mandatory at this point.