Chemical A0-3959X.91 – 15 (Earth-9471)

"The pathogen is protean, changing from liquid to airborne to blood transmission, depending on its environment. It's almost impossible to kill."

- Red Queen

Chemical A0-3959X.91 – 15, also known as Agent A0-3959X.91 – 15, "black liquid", or "Purity", is a strong and potent virulent mutagenic pathogen which consists of millions of micro-organisms. The chemical was manufactured by the ancient forerunner species as a bio organic weapon (B.O.W.) for unknown reasons, which they stored within steatite ampules.

History
The chemical was created by the forerunners at approximately 100,000 BCE on the moon of LV-223 for reasons unknown, but presumably to wipe out the life of entire planets. For reasons unknown, the forerunners were exposed to the pathogen, seemingly extinguishing the entire species with the exception of a lone individual in cryosleep.

In 2293, the USCSS Prometheus arrived on LV-223 to locate forerunners following the discovery of ancient cave drawings on the Isle of Skye, on Scotland. Some of the crew members were exposed to the chemical and became a clicker, killing those who were not. The few survivors sacrificed their own lives, as well as the Event Horizon, to destroy a Juggernaut, while Dr. Elizabeth Shaw and the synthetic David boarded another Juggernaut-class vessel in the search of the Paradise, seemingly the homeworld of the forerunners.

The surviving forerunner was later impregnated by a Tribolite, leading to the birth of athe Deacon. In the aftermath of the disastrous expedition, which included the death of the Weyland Corporation's founder, Peter Bishop Weyland, the subsequently-formed Weyland-Yutani Corporation hid all the information regarding the discoveries made on LV-223, and later uncovered a manual retrieved by an U.S.S. team which concerned the workings of the chemical, the glyphs which were all translated by one of the company's Replicants. The discovery of the document led to the attempted manipulation and weaponization of the chemical as a bio organic weapon by the Umbrella Weyland-Yutani Corporation under Dr. Charles Ashford and Dr. Michael Bishop from 2492 to 2552, who took over the project in the hopes that the groundbreaking scientific discovery would reverse the degenerative illness of his daughter, Angela "Newt" Ashford. His research was primarly conducted at the Hive, a massive underground facility underneath New York City.

Effects
"Bio-readings are...strange. I mean... off the charts strange. Never seen fluctuations like this. Look at this, Francis. Pure chaos. It's like this stuff can't make up its mind what it is."

- James Weddel and Francis Lane Chemical A0-3959X.91 – 15's purpose is that of cleaning planets of any non-botanical lifeform. Once it is exposed to another organism, the pathogen will mutate its host and rewrite their DNA, leading to an uncontrollable physical and mental mutation. Geologist Damon Jeffries, one of the crewmembers of the USCSS Event Horizon, was the first human exposed to the pathogen during the ship's expedition to LV-223 in 2393, which culiminated in his transformation into a Stalker that attacked and further infected his fellow crewmembers.

The infection proccess leads to immediate cellular breakdown within the body, leading to, essentially, the psychological death of the victim, who in turn starts to drip black liquid from their eyes, mouth, and ears, giving them an unhinged hunger for flesh. Duo to her impregnation with a xenomorph Queen, which changed her DNA, as well as multiple failed cloning ventures and eventually clinical resurrection following her death on Fiorina 161 in 2563, Lieutenant Alice Ripley became the only human who has ever been able to bond with the chemical and not die, becoming essentially a superhuman with all the qualities of a xenomorph and an infected but none of the deffects, and thus the only exception to the rule.

Purity has been described by scientists such as Charles Ashford, Michael Bishop, and Mason Wren as a major scientific breakthroughin human history alongside the potential for the xenomorph species and for the Umbrella Corporation, as well as a major profitable gain for the company to sell for military arms, such as the United States Colonial Marine Corps and the Pan-Asian Coalition. When injected into living tissue, the chemical will lead to an uncontrollable physical and mental mutation. Umbrella's research with the chemical, and subsequently its creation - the xenomorphs - was considered illegal by the United Nations and environmentalists, such as Matt and Lisa Addison, who sought to expose Umbrella's work to the world.

While the complete workings of the pathogen are unknown, it is known that its nature if protean, meaning that, once it is released in the environment, it will adapt accordingly, like a virus. According to the Red Queen, Chemical A0-3959X.91 – 15's is both airborne and blood-transferable. A simple scratch or bite from another infected or a xenomorph passes the pathogen from person to person. In the case of animals, ingestion of an infected animal leads one to contract the disease. Much like the Chestburster, an evolved form of the infant chemical, the pathogen attacks the adrenal glands, reaching the spine to the pineal gland in the brain, where it injects a tumor that takes over the nervous system. It causes subsequent violent convulsions and kills the victim.

Stronger infected individuals or stronger xenomorph variants, such as a Praetorian, are incredibly resistant against physical injury, being able to sustain many gunshots. The Dragon had to be drown in molten lead to be killed, while the Axeman took multiple shotgun blasts to die. The pathogen killed several bioraptors. As direct creations of the chemical, xenomorphs exhibit acidic blood and even the ability to regenerate lost body parts, not uncommon on reptiles and flatworms on Earth. Dr. William Weir was exposed to the pathogen and became clinically insane, but was killed by Captain Laurence Miller to prevent him from infecting others. The chemical affected his reproductive system, and before he started to physically become a Clicker, he performed sexual intercourse with his wife, Lieutenant Joely Starck, leading her to be impregnated with a Tribolite despite being infertile. According to David, who started analyzing and studying the workings of the pathogen and xenomorph biology, he concluded that the chemical was designed by the forerunners to infect any non-botanical organisms, which he later demonstrated on Paradise by killing the local forerunner population, killing them outright instead of infecting. If the pathogen infects a viable host, it will gestate a hybridized parasite that will kill them when it emerges, a precursor to the perfected xenomorph life cycle. These creatures, such as the Tribolite and later the Facehugger seek other hosts to attack and impregnate with a xenomorph, whose appearance vary from host organism as a result of the "DNA Reflex" - though they share some common traits, including elongated eyeless heads and acidic blood.