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 exposed to Plasma Grenade radiation.

Sergeant Avery Johnson supposedly has Boren's Syndrome, said to have been 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 can be debilitating. The only way to cure Boren's syndrome is an intensive regimen of Chemotherapy lasting for thirty weeks.

According to theory, some combination of the disease's (apparently) multiple pathologies discourages Flood Super Cells. This would explain 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 that Boren's Syndrome discourages Flood. It may be that the nervous system of someone with the disease simply cannot support Flood. One 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," thus rendering the sufferer immune. However, it is highly likely that if this condition did in fact prevent Flood infection the Forerunners would have found it after hundreds of years of research into Flood prevention.

Trivia

 * Despite Sergent Johnson's supposed immunity to the Flood it is possible to see him infected through a glitch that allows him to survive the cutscene. Johnson also comments during the infection "aren't I supposed to be immune to this?".
 * 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.