Nightmare Begins: When a scientists invent a virus that can transform human contacts to dead faster, he guarded it and make sure there's nothing to find out about his plan to use it to change the world, but when sometimes there's a man that go to the san fierro Zombie Lab he able to meet Dr.Payber to talk about his plan. He suddenly find out what is payber's secret. Dr.Payber killed the man and experiment it. He proove that the virus can transform the Human into Zombie faster so he free it but the zombie attack him and killed him so this is why the whole san fierro are declared into a state of emergency and declared the code red so this is why the virus reach the san andreas too. When a man awakes into a police station he probably it to find out what happens into the whole place and he find out that the whole place is full of zombies and he make his way to survive and fight the zombies in his way

Note: make sure that you dont mind the zombies chasing you and make sure that you're going to wait until your enemies reach the barrel