Attack the infected before he makes his move.
From Create Your Own Story
You move quick, darting at the sick behemoth in the doorway, and jump in his direction when you're just a few feet from him. With both feet you kick out, slamming your heels into his bulbous gut and send him stumbling backwards. As you move to get to your feet, Alisha moves past you and steps out the doorway. Your not sure where she pulls it from, but there's a pistol in her hand. She raises it and pulls the trigger, blasting the infected head into bloody bits.
The blast echoes down the alleyway, and Alisha seems like she's momentarily in shock as the fat man topples backwards and splashes into a puddle.