The Quarantine Zone was a perimeter established by the Covenant upon their landing on Installation 05. The Quarantine Zone was directly inside the Delta Halo Library's Sentinel Wall. The Sentinel Wall, along with the Containment Shield, had kept much of the Flood from spreading, allowing the Sentinels to prevent their spread, but its deactivation by Thel 'Vadamee during his search for the Activation Index allowed the Flood to move freely about the ring.
The area was guarded and patrolled by various Forerunner constructs, ranging from the small but necessary Constructors, to the standard Sentinels and Sentinel Majors, to the titanic Enforcers. The Covenant found a four way fight against the Flood, Sentinels, and UNSC forces, but after the Arbiter Thel 'Vadamee lowered the shield, more and more Flood had begun to spread. The Quarantine Zone was apparently kept starved of all life by the Sentinels. The only signs of past life, besides the Flood, were dead trees. It was constantly snowing in the Quarantine Zone. It seems that the Sentinels or some other systems had caused this climate change in order to prevent life from growing and/or existing in this area, therefore denying the Flood of any possible sources of energy. It is unlikely that the change in climate was enacted by 2401 Penitent Tangent as the Monitor had not sent the requested "security detail" after "2512332 hours, 14 minutes, 6 seconds" (or approximately 287 years) since the initial request for one. Likewise, the presence of trees at all indicates that it was not always such a cold climate, and is instead a response to the Flood outbreak.
Since vehicles had been introduced to the area, the Flood showed the ability to operate these machines and use them against the Sentinels and the Covenant. Most of the vehicles here were Ghosts, Warthogs, a few Wraiths and a few Scorpion Tanks, all brought into the area by the Covenant and UNSC during their advances towards the Library. After acquiring access to heavy military vehicles, the Flood had become far more mobile and much more dangerous.
Since the Human incursion into the area, many of the Marines had been infected by infection forms, converted into Combat and Carrier forms, furthering the Flood life cycle and allowing them to spread quicker. They also allowed the Flood access to a more diverse array of weapons, including Battle Rifles, SMGs, Shotguns, and more, furthering their capabilities in combat.