1x01 Pilot

Hmm, maybe. They showed his syringe, and it seems like they did it for a reason. He might have been sharing needles with his girlfriend, and infected her. But it makes most sense that she OD'ed, died, and turned on her own. Also, when the guy next to him in the hospital died, the doctor seemed to understand that he was going to turn, so the zombie virus was already widespread. Maybe the junky guy will turn out to be immune. There's no evidence of that yet, but it would be an interesting plot twist.