Formula X is the chemical responsible for the zombie outbreaks. All of the House of the Dead games revolve around various outbreaks caused by the formula and the monstrosities it creates. It's proliferation is largely the result of Dr. Curien's obsession with it, as it can cure his son's illness.
The Formula was originally a Supersoldier formula created at the end of the Cold War. However, the associated project was never completed, and the formula would be locked away in a bunker underneath Clement Darling's Prison.
Clement would later discover the formula and attempt to use it to keep his aging mother alive. To that extent he hired the crime boss Caesar to help in this endeavour. Caesar, having ties to the criminal world, had no shortage of bodies to use. He enlisted the help of Jasper Guns, the disabled but incredibly intelligent brother of one of his strippers, Varla Guns. Due to an unknown circumstance, the formula was leaked from the facility and proceeded to infect the house and local townsteads. Caesar, however, cared little and instead forced Jasper to work harder, which resulted in Jasper's attempt at using the formula to gain revenge on Caesar.
After Caesar's link to the formula was discovered, AMS agent G and playboy cop Isaac Washington chased the crime lord throughout the countryside, meeting a variety of mutated townsfolk along the way as well as several of Caesar's "experiments". After chasing Caesar back to the Prison compound, G and Issac would learn of Darling's hand in the matters, but were too late to stop Darling from executing Caesar and transplanting his mother's brain into Varla's body with the formula. The chemical backfired, however, turning Mrs. Darling into a gigantic, mutant beast. G and Issac would later gun down Mrs. Darling and she and her son would perish in the resulting explosion from the bunker.
Years later, Dr. Curien would get his hands on the formula. Backed by Caleb Goldman, Curien would attempt to refine the formula into a cure for his son's illness. However, it only partially succeeded. The process, however, drove Curien insane, looking for a way to create the "perfect being". In his madness he locked all of his staff into the mansion and set loose his abominations. AMS Agents were deployed to deal with Curien, cumulating with a battle with the Magician, a powerful mutant who Curien considered to be "the perfect being". The Magician, however, turned on his creator before being gunned down by the agents, resulting in Curien's research ultimately ending in failure. During this time, the chemical seemed to have no virulent properties, and all zombies seems to have been directly created by Curien, either as experiments or failed versions of the Magician.
After the Curien incident, Goldman would use the formula to change the world and give it a new order. Setting it loose on his city, the AMS agents would again arrive to stop him. The undead would run rampant through the city killing everyone in sight. Goldman had planned to cleanse the world of humans, and instead have the successor to the Magician, the Emperor, rule over the new, humanless world. Like Curien, his plans would fail when the AMS agents gunned down the Emperor. Goldman would commit suicide as a result, but not before leaving behind a legacy.
Years after the Goldman incident, two AMS agents in Venice would be besieged by another undead outbreak, caused by Goldman post-mortem. Goldman, through a series of recorded videos, reveals that while the Emperor reached completion during the first Goldman incident, the World was still under development at the time, and was to coincide with a nuclear attack. Through an unknown means, Goldman unleashed another outbreak on the city, led by the Star, a creature similar to both the Magician and the Emperor. AMS agents would again be deployed to stop him and his creations, but not before the sacrifice of one of the agents.
AMS, however, was unable to contain the infection this time, and similar outbreaks around the world destroyed civilization. All clues pointed to Dr Curien's research laboratory, which was designed to revive the dead scientist as a superior being.
Formula X was originally designed to be a super soldier formula, and lingering effects of this is the zombie's inhuman tolerance for pain, being able to function without limbs and, in some cases, without parts of their heads. True to it's purpose, the creatures it creates can be incredibly strong and durable, several of which can be completely immune to gunfire except for small, exposed organs, and can take quite an amount of punishment before going down. Others require severe trauma to be put down, and even then the chemical has proven to be able to revive the subject (with memories intact, should the subject be sentient). The chemical can cross species, frequently infecting fish, birds, bats and even worms (the latter of which develop extremely sharp teeth and grow to enormous sizes). It provokes aggression in all subjects.
The Chemical seems to have mutagenic properties as well, as it frequently combines the traits of various creatures into one. It also causes exponential growth within it's subjects, as several bosses grow to gargantuan sizes. Deterioration of the mind is common, although some subjects retain higher brain functions (such as Sebastian and The Star), all subjects seem to be driven insane. Certain mutations are pre-programmed with loyalty to their creators, although this does not always work. Some mutations develop telekinetic powers, able to send energy blasts and move objects with their minds. Certain mutations borderline magic, such as those of the Magician, Star and Screamer.