From Halopedia, the Halo wiki
The Blood Sickness is an illness that afflicted Ussan Sangheili of the Refuge. Those afflicted with it would become violently insane, while attracting followers through ranting speeches. During the last century of the Broken Circle, Ussans who were Blood Sick would lead uprisings against the other clans, creating dissent throughout the colony.
The first signs of Blood Sickness were malaise and dizziness, which were easy to mistake for milder flus. In the second phase, the Blood Sick became paranoid and frenzied, frequently spouting loud speeches full of raving rhetoric about ills of the world. They would also experience hallucinations, which would be mistaken for visions. In the third phase, the Blood Sick would possess webs of scarlet pulsing under their faces and hands. The final stage of the disease then led to immense pain before a slow death.
Despite their obvious madness, Blood Sick would attract followers through their speeches, either from other Blood Sick or Ussans who felt dissent towards their current leaders. Factions would form around their "philosophy", which could result in violent uprisings. Attempting to approach or reason with them could lead to violence. Amongst themselves, though, the Blood Sick could be surprisingly calm and coherent, and would chose a leader among each other. Those who were Blood Sick were either killed or taken to the Isolation Ward, as before late 2552 there was no known cure.
Blood Sickness could not be passed on by infection, and originated from genetic sensitivity to tainted strains of food from the colony's protein synthesizers. Those who were weaker to the virus began the lapse into Blood Sickness.
The Blood Sickness originated from a flaw in the Refuge's protein feeders, which began to deteriorate in the millennia and a half that the facilities went without any repairs.
During the rule of Kaidon N'Zursa 'Xellus, a Blood Sick group formed called the 'Greftus Faction, led by 'Insa 'Greftus. The group voiced anger over the decay of the Broken Circle and declared that their worship of the "Forgotten Gods", deities that 'Greftus had imagined through his hallucinations. 'Greftus claimed the gods told him that the Ussans had to leave to Sanghelios, and had given him a vision with directions to the homeworld. The Greftus Faction attempted to storm the Primary Section to gain control of the ships there, but they were stopped by N'Zursa, who later had 'Insa 'Greftus executed.
Another faction, the Blue Mandibles, arose during the days of kaidon Bal'Tol 'Xellus. In Section Two there was a food shortage, and the Ussans ill with Blood Sickness there came to believe that they had to resort to cannibalism. They proceeded to attack and devour "lesser" Ussans, earning the group name "Blue Mandibles" because of their bloodstained jaws. Bal'Tol put down the group with his warriors, and years later was still unnerved by the memory of them.
Bal'Tol later fought another group, led by 'Kinsa, who with his followers worshipped the "Forgotten Gods". An attempt to subdue him failed, and 'Kinsa ended up taking over Section Five, ruling over its inhabitants with an iron fist. Worried that another attack might kill innocents in the crossfire, Bal'Tol challenged 'Kinsa and his followers to a floatfight to lure him and the afflicted out. His team was defeated and 'Kinsa killed shortly thereafter by the newly reactivated Enduring Bias. With his help and the aid of Huragok Sluggish Drifter, the protein synthesizer was repaired and an antitoxin was developed to vaccinate all against future Blood Sickness.
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