Boren's Syndrome

Boren's Syndrome is a human disease caused by prolonged exposure to high-yield plasma or electromagnetic radiation, such as from a Plasma Grenade. Symptoms include brain tumors, migraines, and amnesia. Without proper treatment, death commonly occurs. Effective treatment requires thirty weeks of intensive chemotherapy. Unconfirmed claims as to the cause of the disease include inhaling the gases released when a Type-51 Carbine magazine is ejected and being around Plasma Grenades and being exposed to their radiation.

Sergeant Avery Johnson officially has Boren's Syndrome, supposedly contracted when he used an entire crate of plasma grenades to hold off Covenant forces on Paris IV, though this story is apparently a hoax (known as the Paris/BS Spoof) used to hide that Avery Johnson could be a SPARTAN-I.

Several members of the United Rebel Front claimed to have this condition, hoping to bargain with the UNSC to gain treatment for the condition in trade for some FENRIS Nuclear Warheads they had in stock.

Long Term Effects
Untreated or unsuccessfully treated, Boren's can be fatal or debilitating. While some effects, such as migraines, can be managed with medication, the tumors involved in the disease usually, or possibly always, become malignant. Because of this, treating Boren's Syndrome invariably means treating cancer, a process that itself can be debilitating.

Ironically, some combination of the disease's (apparently) multiple pathologies discourages Flood Super Cells. This explains why Sergeant Avery Johnson could not be infected when he and his squad were ambushed by the Flood on Installation 04, leading Master Chief to his moral dilemma. Dr. Catherine Halsey guesses that there'd be a billion to one chance that Johnson's condition could be reproduced.

Technically, there is no explicit proof of what about Boren's Syndrome discourages Flood. It may be that the nervous system of someone with the disease simply cannot support Flood. A strong hypothesis is that an infection form attempting to latch on to a host with Boren's Syndrome would recognize the characteristic neurological disturbances as "irreparable damage".

Trivia

 * Despite Sergent Johnson's immunity to the flood, through an oversight in Halo 3, it is possible to have him infected by flood.
 * Since Johnson is programmed to revive after being killed, stopping an infection midway will result in momentary death, suddenly changing to a marine infected model and getting up.
 * Johnson actually has dialog for infection sequences, including This was for Hocus!