(Capcom's primary storyline)
"Uroboros will be released into the atmosphere, ensuring complete global saturation!"
Uroboros was created by Albert Wesker with assistance from Excella Gionne, CEO of TRICELL, Inc.'s African division. Although it is likely that elements of other viral agents were used in the creation of Uroboros, two factors were of particular importance to the advancement of this new pathogen: the plant known as the "Stairway of the Sun", native to the African region of Kijuju, and Jill Valentine, who was presumed dead on November 23, 2006 after an encounter with Albert Wesker at the home of Oswell E. Spencer.
"Stairway of the Sun" in local legend, played an important role in ceremonies conducted by the native aborigine tribe of Kijuju, the Ndipaya. Since ancient times, the monarch of the Ndipaya was chosen not by birthright, but through selection by the demonstration of abilities and qualities of chosen tribesmen during a special ceremony. The flower was an extremely poisonous plant, but the Ndipaya believed that one who survived consuming the plant was worthy of becoming their king. Individuals who could survive the poisoning were extremely rare. It is said by the Ndipaya people that one such man reigned as king for centuries, though the validity of this account remains unconfirmed.
The first foreigner to discover the flower was Henry Travis, an explorer who traveled throughout much of Africa for 35 years during the 19th century and a scion of the Travis family, the future founding family of TRICELL. Henry compiled all of his discoveries in the 72-volume encyclopedia set on the African continent, Natural History Conspectus. These books contained detailed records from animals, plants, insects, minerals and topography to the native inhabitants and their cultures, folklore, histories and traditions. Oswell E. Spencer, who read the Natural History Conspectus extensively, was particularly interested in the Ndipayan rituals and their use of the flower. Convinced of the significance of the plant, Spencer resolved to find the plant and use it to somehow accelerate the evolution of humankind.
In the 1960s, Umbrella, under the direction of Spencer, occupied the ancient ruined city of the Ndipaya to find the flower, despite the fierce initial resistance of the Ndipaya descendants who now lived in the nearby wetlands. As a sidenote, many young Ndipaya warriors consumed the flower in an attempt to help fight off Umbrella, suggesting that perhaps the flower already had a sort of physical enhancement property. Eventually, Umbrella forced the Ndipaya to cede the area where the ruins are located to their control. Umbrella then set up a secret laboratory within the ruins and began experimentation on the flower. Eventually, the Progenitor virus was derived from the flower by Dr. James Marcus and his research team on December 4, 1966.
Spencer fully intended to protect the secret research on the flower so he could always begin anew if Umbrella were to be dissolved. Knowledge of the existence of the African facility was restricted and very few employees at this facility were transferred to work elsewhere. Those who were transferred were carefully monitored and Umbrella maintained a strict company policy in controlling the flow of information. The facility was officially closed during the 1980s with its research and equipment intact. Most of those who worked at the facility, including Marcus' former research assistant, were secretly murdered as well. In this way, the Umbrella African facility remained unknown to the world at large with its research preserved from those who would destroy it.
TRICELL learned of the African facility while tracking down senior Umbrella employees with Level 9 and Level 10 security clearance. Surviving his encounter with Chris Redfield and Jill Valentine, Wesker sought to bring the Umbrella African facility back into operation with help from TRICELL executives who were also interested in claiming Umbrella's viral legacy.
Wesker began to pour all his collective resources, viral or otherwise, into the goal of creating a new pathogen (which would later be dubbed as the Uroboros virus) that would start the next step of human evolution and scour weakness from the world, based on Umbrella's previous research into the Progenitor flower. Although badly wounded and unconscious, Jill was saved by Wesker. After giving her the medical treatment she required, he placed her in a cryogenic sleep. Wesker intended to use Jill as the first test subject for the completed Uroboros virus - an act of vengeance against his hated enemy, Chris Redfield.
The development of Uroboros encountered problems early on. In most cases, the Uroboros virus proved too fatal to humans for Wesker's purpose. Instead of accelerating human evolution, Uroboros only brought a swift death. During this time, the apparatus monitoring Jill's vital signs detected some abnormalities. Further investigation showed that a mutated form of the t-Virus was still inside her body - a remnant from her t-Virus infection by the Nemesis-T Type in Raccoon City. The cure she was given was supposed to have eradicated all traces of the virus in her body, but it instead caused the virus to go into a dormant state. Her extended period in a cryogenic sleep somehow reactivated the virus. Shortly after being reactivated, the t-Virus completely disappeared from her body, but it left something else in its place. Wesker discovered that Jill's body now contained powerful antibodies. He then theorized that using Jill's antibodies could make Uroboros less poisonous, thus allowing potential carriers to live long enough as usable bio-weapons. He kept Jill alive solely to produce antibodies for his research. For someone such as Jill who dedicated her life to the elimination of bio-weapons, it was ironic that she would be used in the development of one of the deadliest bio-weapons yet.
After much research and experimentation, Wesker finally perfected the Uroboros virus. Jill's participation in its development meant she was no longer suitable as a test subject. Pure and unadulterated antibodies with high resistance to Uroboros permeated her body. However, Wesker would find an alternative purpose for her.
Unfortunately for Wesker, Chris Redfield and Sheva Alomar soon discovered his plans. After defeating numerous mutations created by the Uroboros and rescuing Jill from Wesker's control, they killed Wesker, eliminating the threat of an Uroboros outbreak on the world.
Sometime in 2011, the Uroboros virus was used by Alex Wesker in Zabytij Island to create B.O.W.s like the Revenants. She also injected Neil Fisher in his neck with the virus after he attempted to obtain a sample of it from her in exchange for supplying test subjects for the fear experiment. Neil's mutant form was killed by Moira Burton and Claire Redfield.
Six months later Alex injected herself with Uroboros virus in order to overcome her fear of confronting her "other self" within Natalia Korda. She was later killed by Claire who shoots her with a single rocket from her RPG-7 while in a helicopter along with Moira and Natalia.
In its original form, Uroboros was too deadly for any host to support life. Only with the introduction of Jill Valentine's t-Virus antibodies could Uroboros' rapid amplification be controlled so that it left a select percent of the human population alive.
Once Uroboros comes into contact with an organism's cells and infects them, it inserts its genes into the host's genome in an attempt to merge with the host. For those whose genetic sequence is compatible with Uroboros, the merge is successful, bestowing superhuman abilities upon the host. Those whose genomes are incompatible fall to the virus' uncontrolled amplification, and their cells are overtaken by Uroboros. The resulting infected tissues, organized into undifferentiated tentacles, absorbs the body of the host and any other organic matter it can reach. These tentacles are held together by yellow pustule-like organs, which are its only weak-point.
When a human host successfully adapts to the virus, they retain their normal human form but become superhuman with increased strength, speed, durability, vitality and a strong regeneration ability. The only physical sign of mutation in an adapted host is the appearance of glowing slitted eyes, similar to those of a host successfully adapted to the Progenitor virus or the prototype t-virus variant Albert Wesker adapted to. However, due to its purpose of forced evolution, it was designed to accept a small percentage of the human population possessing excellent genes. If Uroboros had been spread across the globe, it would have killed those with unsuitable DNA and selected for genomes containing genes favorable to evolution.
Uroboros was sensitive to intense heat as shown in all encounters with Uroboros, also after Wesker's bonding with Uroboros and falling into the lava he was unable to attack Chris and Sheva, only being able to launch one of his arms at the helicopter Josh piloted. The heat generally comes from places like furnaces and lava pits, although any heat source could work, as evidenced by Neil Fisher's Uroboros-infected form being vulnerable to the heat generated by a flashlight.
Not much else can be said without extrapolating information, since there have been only four hosts seen in the games, three of which were incompatible. Because Wesker's situation, with his preexisting virus and possibly the overdose on PG67A/W, is so unique, it is difficult to say if the symptoms he showed were typical of all compatible hosts, or if they were peculiar to him, though saying Wesker is typical is not the best description (he might have been bonding, but was killed before it completed). It is possible that Uroboros-infected cells, with or without a compatible host, develop a form of sentience, or at least characteristics of an individual organism. The effects shown by Wesker was the ability to use electricity with the metal he picked up (although it might be because of friction), and he had increased jumping ability with increased resistance of damage, regeneration, and strength (although he was overdosed with PG67A/W which made him weaker and poisoned from the inside).
- Although never referred to as such in dialogue, two of the three Uroboros creatures fought in-game have specific names: The one created in the laboratory was called Uroboros Mkono, and the one "birthed" from Excella Gionne was called Uroboros Aheri.
- It is unknown if Wesker's theory was true of the Uroboros being the Philosopher's Stone by selecting humans that are "worthy" enough to survive the merging process as there are currently no other human that had survived being merged by the Uroboros aside from Wesker himself, although he might have gained immunity from the Uroboros through the virus that Birkin gave him which granted him his powers. It was later revealed in Resident Evil Revelations 2 that this theory is true, since Neil Fisher also seems to have survived the merging process as well, retaining most of his awareness and becoming very powerful being.
- In a trailer for the rejected 'Castle' version of Resident Evil 4, Leon is seen fighting a black tentacle mass which bears a similarity to Uroboros. The creature is officially dubbed as "Black Fog" by the developers and was created by Albert Wesker and released by his special forces known as the H.C.F.