Sgt. Johnson survived Halo 1 after being killed (at least once, if not more) and was not assimilated by the Flood because of Boren's Syndrome, a disease he contracted from hanging around too many plasma grenades. Johnson declined treatment for his Boren's Syndrome to get back into battle more quickly. The effects of the disease on his nervous system led the Flood infection form to reject his body. Boren's Syndrome is named after Brannon Boren, keeper of the Halo Bible and, as of this writing, working on Matrix Online. Johnson is shown here with a battle rifle. In Halo 2, you can exchange weapons with the Sarge or other human units.