Chemical A0-3959X.91 – 15 (as designated by Weyland Corporation) was a lethal pathogen (actually a mutating agent), composed of millions of small micro-organisms, that was manufactured by the Engineers, most likely as a biological weapon for war and genocidal purposes. Due to its unstable nature at ambient temperature, the Engineers housed the pathogen within Steatite ampules.
The pathogen can infect organisms through a multitude of means, with a variety of ourcomes, including:
- Inhalation: causes a series of stages from confusion/shock, cellular mutations and DNA recoding, to advanced dementia and instinctual aggression after 50 hours.
- Ingestion: causes onset of sepsis within 1 hour, then paranoia/delerium, convulsions/internal bleeding, to DNA disintegration after 50 hours.
Geologist Fifield was exposed to the pathogen during the USCSS Prometheus' expedition of LV-223, mutating his appearance and causing him to attack his fellow crew members. He was also extremely resistant to physical injury, being able to withstand multiple gunshots. It took several shotgun blasts and being scorched with a flamethrower to finally kill him. The pathogen also mutated several worms found in the Engineer temple, turning them into the aggressive Hammerpedes. The Hammerpedes exhibited acidic blood and the ability to regenerate lost body parts, though the latter is an ability commonly found in Earth worms. However, the regeneration of Hammerpedes is much faster than that of normal Earth worms, suggesting that the pathogen can enhance a host's natural abilities.
Holloway was also exposed to the pathogen and began mutating before Meredith Vickers killed him to prevent the infection from coming aboard the Prometheus. The virus appeared to affect his reproductive system. Prior to his physical mutation (while carrying the virus), Holloway had sexual intercourse with Elizabeth Shaw, impregnating her with the Trilobite. It is not known if his sperm was mutated causing an abnormal conception or if he was simply carrying a Trilobite that moved into Shaw, though the latter seems more probable given that Shaw was sterile. Whether Trilobites prefer to move into female subjects or if Holloway would have naturally given "birth" to the Trilobite is unknown. Although Shaw was exposed to the infected Holloway and had intimate contact with him, she was not infected with the liquid and only received the Trilobite embryo. This suggests that the black liquid itself is not sexually transmitted, although it can affect the host's reproductive systems.The Trilobite carries a lone creature egg for impregnation.
It was shown that the Engineers either created or were housing the black liquid thousands of years ago on LV-223 for the purpose of unleashing them on their descendents on Earth. Somehow, the Engineers themselves were exposed to the pathogen, wiping out the entire Earth invasion force, except for one still in stasis.
In 2093, the USCSS Prometheus came to explore LV-223 after finding clues of the Engineers' existence in ancient cave drawings on the Isle of Skye. A few crew members were exposed to the pathogen, and in the ensuing chaos most were killed. The remaining crew later sacrificed themselves and the Prometheus to destroy the Engineer's ship, with only Elizabeth Shaw and the android David as the only survivors leaving with another abandoned ship to head for the Engineer's homeworld.