Bob is an NPC encountered in the Outbreak scenario. He is already infected with the T-Virus and requires the player character's assistance in escaping due to his weakened state, though it is up to the player if they wish to try and escort Bob to safety or not.
Once reaching the roof, Bob will then either commit suicide to halt his zombie transformation, or will in fact turn into a zombie. Bob will only turn into a zombie during the scenario's Very Hard difficulty setting, however.
Bob can be unlocked as a character skin set over the Mark Wilkins character by completing the scenario with Mark. Though his condition will always start at Danger, he at least starts off with 10 handgun bullets in his inventory.