The Plague spent most of its early history on the planet in deep hibernation, it’s existence concealed from the human population by the select few who knew of its presence. One of these individuals was their discoverer, Axel Capek, lead scientist of the Ultor Corporation who, with his Martian Advanced Science Team (MASTers), explored an ancient shaft that seemed to have been carved deliberately by some unknown precursor race originally native to the Red planet.
He speculated that the Plague, like humans, were not native to the planet, but were an invasive species that had been introduced at some unknown historical point. The Seal which kept the Plague in hibernation acted to control the environment within the caves; extremely advanced technology that was far beyond the current capabilities of the human colonists, even by the time of its destruction by Darius Mason.
Capek's scientific discoveries were a direct result of what he found down there, leading to the creation of nanotechnology and the Nano Forge, but he mostly experimented with samples of DNA taken from the Plague, creating hybrid mutants by splicing them with humans and various other creatures. These were referred to as the Ultor Plague of 2075 and were destroyed after the conclusion of the First Martian Revolution.
The Plague themselves made their first real appearance in the Plague Armageddon, let loose by the Cultists, a splinter group of the Marauders and parts of the remaining White Faction, led by Adam Hale.
The Plague was later destroyed after Darius repaired the Terraformer, making the surface habitable for humans but deadly to the Plague, suffocating them with a newly restored Earth-like atmosphere. It is possible however that there are still Plague hives hidden deep underground, evolving, adapting, waiting for their chance to attack.
The majority of the Plague are Insectoid in appearance. However, some appear to have humanoid features, perhaps as a side effect of consuming and absorbing human DNA. The most notable part of their appearance is that their elbows, knees and body spikes are bioluminescent, much in the way of earthly deep sea fishes. All Plague have their own unique abilities that usually consist of ranged or close-quarters attack, such as acidic bile or claws. Most Plague are taller than humans, with the exception of the creepers.
- Creeper: The least intelligent of the Plague and the easiest to kill. There is also a larger variant which has similar attack patterns as an ordinary creeper.
- Ravager: A more intelligent creature of the Plague. Ravagers mostly dispatch their prey from long-range.
- Wraith: Hit-and-run creatures who appear to shoot their prey with a tri-laser attack and quickly disappear if attacked. They use unknown microscopic organisms to disorient their prey.
- Berserker: Given their name, Berserkers often shoot at their enemies with a highly concentrated blast that has a big splash effect. When near death, they rush toward their prey and explode if they get close enough.
- Behemoth: The second biggest variant of the Plague. Behemoths are powerful enemies that can shoot at their prey with highly concentrated blasts like the Berserker.
- Pod: Incubators that produced more plague inside the egg sac.
- Tentacle: Tentacles can grab nearby debris and throw it at their prey. They also have a close range attack if their prey comes too close.
- Monolith: Monoliths appear out of the ground, providing a boost of endurance to any of the creatures in proximity up to 50% of their defense.
- The Queen: The leader and possibly the breeder of the Plague. She is presumed to have escaped, even though Darius had damaged her to a serious extent.
To the miners on Mars, during the First Martian Revolution, the plague was a mysterious force killing miners and creating grotesque mutants, but that is all that is known, its alien origins completely unknown to the miners and the first Red Faction rebellion. They attack the player from time to time in the first game and also serve as two bosses, one of which is optional. Upon meeting mutated miners, Axel Capek's voice will come make taunting remarks along the lines of, "See any old friends, Parker?" These are not known to be aliens and were thought to be mutants in the first game. The mutated miners ARE mutants for sure, the rest are not confirmed to be.
- Reapers: A.K.A. "rat-monkeys" They come in several sizes and run after the player. They are fairly easy to kill and don't pose much of a threat alone, but are often found in groups of 2 or 3 and can swarm the player and deal with you if you don't kill them soon. These share many similarities to creepers from Armageddon in body structure and behavior, although they don't fire projectiles. The larger Creeper is closer to the same size. They do not jump however, but are extremely un-intelligent.
- Sonar Whales: Found underwater, they greatly resemble a whale, giving them their name. They shoot sonar waves at the player. These sonars cannot leave the water and simply cause a splash effect upon hitting the surface. Use your pistol to kill them underwater as no other weapons can even damage them
- Mutated Miners: These are brown and sometimes appearing obese, they run after the player, and as said before, usually when finding some, you are greeted with Capek taunting you.
- Giant Worm: They serve as a boss in the game and upon being killed, fall over and if they land on you, can kill you. Don't go near it or it kills you instantly. It is extremely vulnerable to fire so if you have the Flamethrower, just half a second is enough to instantly kill it. If you didn't pick up the Flamethrower near the Water, Just stride left and right and fire constantly into its mouth.
- Baby Giant Worm: A slightly smaller worm, you can fight this, or cross a bridge full of guards. Just use the same strategy as the other worm. This one uses the same attacks and should not be approached closely.
- The Plague are referred to as "Legion" in the game files.
- It is possible that the Plague breeder of the Queen as female, that counted all its kinds are female.