(Capcom's primary storyline)
"Uroboros will be released into the atmosphere, ensuring complete global saturation!"
Uroboros (ウロボロス) is an RNA virus in the Progenitor family. Engineered in the 21st century by Dr. Albert Wesker as the ultimate eugenics tool, a less potent strain was later developed by Dr. Alex Wesker to serve as a bio-weapon. There is currently no known vaccine for either strain of Uroboros. The virus' name is derived from the Ouroboros in Greek mythology, a snake or snake-like creature which consumed itself.
Work began on the Uroboros Project sometime in 2006, following an encounter Dr. Albert Wesker had with Dr. Oswell E. Spencer, Earl Spencer, who revealed to him Umbrella Pharmaceuticals' secret eugenics goal in creating a race of Übermensch to advance human society, with Wesker himself being part of such a project. Having begun to question the emerging War on Terror, he concluded that humanity itself was the problem, and the only way to bring about a utopian society of the Übermensch was to use the Progenitor Virus against the very planet itself, which would kill all but those with certain genetic characteristics. Such people would theoretically be enhanced physically and mentally.
The first set back to take place after Tricell - Wesker's partner - set up a lab in the old Ndipaya Kingdom, was that the human population ratio able to survive and mutate from Progenitor infection was simply too small. Using t-Virus antibodies extracted from a mutant test subject, Jill Valentine, Wesker repeatedly used them on his Uroboros cultures to naturally select a new strain distinct and less potent than Progenitor. With a virus now useful in research, the project moved to its next stage and saw thousands of other test subjects infected over the next two years, with increasing assistance from Tricell's own research staff such as Miguel. The virus was more-or-less finalised by May 2008, when samples were loaded onto missiles in preparation for the final stage in the "Uroboros Project" - dispersing the virus across the Earth's atmosphere to infect every corner of the planet. There were problems with the preparations, however and a fire occurred, resulting in the creation of a mutant arthropod species dubbed "Reaper". In March 2009, the project was finally underway, but was stopped by the Bioterrorism Security Assessment Alliance. At least four humans were infected with Uroboros during the operation, including Wesker himself. Only he mutated appropriately, however, with the other three being genetically incompatible. In their case, their bodies were torn apart by uncontrollable mutation, and in one case consumed a number of other bodies.
"Uroboros... It's not death... it's power!"
— Neil Fisher while mutating
Sometime after Albert Wesker's death, his compatriot Alex Wesker managed to obtain a sample of Uroboros. Taking it to Sejm Island, bio-weapons research began shortly after work on the t-Phobos Virus was completed. The injection of Uroboros into the corpses of failed t-Phobos test subjects resulted in the Revenant. In 2011, Alex Wesker conspired with TerraSave leader Neil Fisher to provide Uroboros in exchange for more t-Phobos test subjects. Fisher was instead infected with the virus rather than handed a sample, Alex's punishment for him betraying his friends. Upon her own mutation from Uroboros and t-Phobos, Alex released her Revenants from the laboratories and across the island. Over the next six months they slowly consumed every last food source from corpses to wildlife. They were eventually eradicated by a BSAA team. During the time when both Barry Burton and Claire Redfield along with Moira Burton (Barry's daughter who been kidnapped) confronted a horrifically mutated Alex Wesker, and severely damaged her, she consumed large doses of Uroboros, augmenting her and making her enormous in size. At the end, Claire destroyed Alex by firing an RPG right into her exposed heart, ending her life, experiments and reign of terror.
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 percentage 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, organised into undifferentiated tentacles, absorbs the body of the host and any other organic matter it can reach. These tentacles are held together by orange pustule-like organs, which serve as the control center of the organic growth. Destruction of the core negates the body's cohesion, and thus, life. The same happens when the virus is injected in any corpse.
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 mutants remain sensitive to intense heat as shown in all encounters with Uroboros, and 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 five 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 Albert Wesker's mutation is consistent with his expectations. As a mutant, he was already genetically distinct from the population the virus was intended to infect. Neil Fisher was also infected with the t-Phobos Virus, which may explain his own unusual mutation, though this was a separate Uroboros strain.
- 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. In the story: It was created by Albert Wesker (said to be a combination of multiple viruses) and released by his special forces team, known as the H.C.F.
- The trailer for non-canonical Wesker Mode for the HD Remaster of Resident Evil 0 mentioned Uroboros, where it implied that Wesker was empowered by the Uroboros virus during the spin-off game mode.
- Uroboros is likely named after Ouroboros, a mythical serpent (or dragon) that devours its own tail. The ouroboros often symbolises self-reflexivity, introspection, or cyclicality, especially in the sense of something constantly re-creating itself, the eternal return.
- Resident Evil 5 (2009), file: "Uroboros Plan".
- Resident Evil 5 (2009), file: "Albert Wesker".
- Resident Evil 5 (2009), file: "Jill Valentine".
- Resident Evil 5 (2009), file: "Regarding the Mutant Organism Found in the Facilities"
- Resident Evil: Revelations 2 (2015), file: "Research Facility Chief's Journal 2"
- Resident Evil: Revelations 2 (2015), file: "Researcher's Journal".
- Resident Evil: Revelations 2 (2014), file: "Neil's Candidate List".
- Resident Evil 4 prototype