The Black Liquid is an extremely lethal pathogen composed of millions of small micro-organisms. Once exposed to another organism, the virus begins mutating its host. The host becomes extremely aggressive and seemingly mindless as it attacks any living thing in sight. Geologist Fifield was exposed to the liquid during the Prometheus' expedition of LV-223, mutating his appearance and causing him to attack his fellow crewmembers. 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. He was also able to survive on LV-223's surface without a helmet for extended periods of time (despite its heavy carbon monoxide content, capable of killing a human within a few minutes), showing superhuman endurance and possibly modifications to his respiratory system. The liquid also mutated several tiny worms found in the Engineer ship, 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 and thus may not be caused by the black liquid. However, the regeneration of Hammerpedes is much faster than that of Earth worms, suggesting that the Black Liquid 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 virus from coming aboard the Prometheus. The virus appeared to affect his reproductive system, as prior to his physical mutation (while carrying the pathogen), Holloway had sexual intercourse with his girlfriend Elizabeth Shaw, impregnating her with a 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 Deacon egg for impregnation.
It appears the black liquid is intended to mutate its target populance, turning the afflicted against one another and mutating indigenous lifeforms into hostile xenoforms before finally unleashing the Deacons on the weakened population. However, the full effects of the pathogen are unknown and may vary from subject to subject, as shown by Holloway and Fifield. Infected hosts seem to share some characteristics with the Xenomorphs, such as unnatural hostility, endurance and, in some cases, acidic blood.
In Prometheus, it was shown that the Engineers created and/or housed the black liquid two-thousand years ago on LV-223 for the purpose of unleashing them on their descendants on Earth. Somehow, the Engineers themselves were exposed to the pathogen, wiping out the entire invasion force except for one still in stasis. The Engineer race then abandoned the moon.
The crew of the USCSS Prometheus came to explore the planet after finding clues of the Engineers' existence in ancient cave drawings. A few crew members were exposed to the black liquid, 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 robot David leaving with another abandoned ship for the Engineer homeworld.