Johnson enlisted in the UNSC Marine Corps when he was nineteen and was soon thrown into the Insurrection.His name was dubiously associated with dozens of black operations and programs, including assignment to the highly classified unit called ORION Project, which performed "enhancement" procedures on volunteer troops, who would later be referred to as SPARTAN-Is. Johnson was also known to have participated on missions codenamed TANGLEWOOD and KALEIDOSCOPE.
A query by the A.I. Cortana into Johnson's medical history revealed that he had a false condition known as Boren's syndrome. This disease was supposedly contracted while he was stationed at Paris IV. When the Covenant attacked Paris IV, Johnson fought valiantly; during the battle, he repossessed a crate of Covenant plasma grenades when his troops needed them and used them all against the enemy troops to keep them at bay so his fellow soldiers could evacuate. He received a commendation for bravery, and a twelve-hundred-rad cumulative dose of radiation as an unanticipated "bonus" from all the plasma detonations.
However, the Boren's syndrome apparently had an unexpected side effect, as it made him effectively immune to infection by the sentient alien parasite known as the Flood because the radiation scrambled his nervous system so much that Flood Infection Forms could not force a match. This rendered the Flood unable to control him. However, the Boren's syndrome story (referred to as the "Paris/BS Spoof"), was a hoax to cover up Johnson's involvement in the ORION Project, and that his immunity to Flood infection is a result of the procedures performed on him to make him a SPARTAN-I .
Taken from halo.wikia.com
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