You really believe that? He was part of the Spartan 1 project, code named ORION. They were the first super soldiers put into service by the UNSC. They participated in Operation Trebuchet. The Illness Johnson has, Boren's Syndrome, was made up by ONI to cover up ORION after it was shut down. Soldiers that experienced its symptoms were candidates of ORION integrated into the UNSC Armed Forces. Their modifications enabled them to have immunity from the Flood. I'll post a few links.