- last post: 01.01.0001 12:00 AM PDT
He escaped like this, Sergeant Johnson survived the explosion of the first Halo because (as detailed in the Halo novel First Strike) he escaped the explosion in a Pelican dropship with a small group of survivors. The Pelican hid in the debris of Halo before being found by Master Chief and Cortana whilst they were still flying in a Longsword fighter. As for how he survived his encounter with the Flood in the first game, the novel First Strike details how the Sergeant has a rare medical condition called Boren's Syndrome, which results from a heavy exposure of the nervous system to the electromagnetic fields emitted by Covenant plasma grenade detonations. Sergeant Johnson obtained this condition when he used a crate full of plasma grenades in a past conflict against the Covenant during the Siege of Paris IV. This condition is what saved him from the Flood's ability to consume their hosts' minds, as the abnormal frequencies in his nervous system served as a buffer to keep the Flood out of his mind.
In Halo 2, he was awarded the Colonial Cross, led UNSC forces to drive the Covenant from New Mombasa and Delta Halo and was captured along with Miranda Keyes by the Covenant but managed to escape when the Arbiter arrived and attacked his captors. He took advantage of the distraction, allowing him to commandeer a nearby Scarab and request the Arbiter's help in defeating Tartarus.
At the end of the game, 343 Guilty Spark takes him to the floor of the Control Room and is about to argue with him when Keyes asks the Monitor about a place where all the Halo installations can be activated; the Ark.