(Capcom's primary storyline)
The Animality Virus, or simply the A-Virus, is a genus of Progenitor virus which was engineered as a bioweapon by a research team led by former CIA agent Glenn Arias. A-Virus is the cause of a disease similar to that of the t-Virus caused Cannibal Disease, wherein infected humans - dubbed "Zombies" - experience violent tendencies, increased hunger to the point of cannibalism, and deterioration of cognitive function. What sets this viral disease apart is the slower rate of cognitive deterioration, with Zombies capable of a degree of speech and coordination.
When this viral research project began is presently unknown, though likely took place following the demise of Tricell. The research project was ultimately led by Glenn Arias, a former CIA agent who turned his coat after gaining influence in the bio-weapons black market he was sent to infiltrate. Following a failed drone strike intended to kill him, Arias began plans to trigger a massive biohazard in the United States.
During the research, Arias was able to broker a deal with Spanish terrorist group Los Iluminados, who provided a Plaga sample for testing. Selecting the genes responsible for the parasite's control over the nervous system, Arias' team was able to create a modified t-Virus that would mutate hosts to become social creatures similar to Ganados, while outwardly behaving the same as the Zombie victims of Cannibal Disease. Ultimately, three strains were created to create a reliable weapon. The standard Type I strain which would be dormant until triggered by Type II infection, and a Type III strain that would mutate the host to possess the Plaga genes but not show Zombie effects, preventing the user from becoming prey.
The A-Virus' Latent strain was dispersed into The Great Lakes by Glenn Arias, due to the number of bottling refineries that distribute its water supply across America. To ensure the Latent strain was thoroughly spread, Arias created a dummy company, A-GUA Industries, Ltd. as a front to further distribute contaminated water. This dormant virus caused unprecedented mutations around parts of Canada and the USA due to rare instances of the Latent strain activating without the Trigger Virus.
The first large scale outbreak of the A-Virus was conducted by Glenn Arias, who planned to release the airborne Trigger strain upon New York City via Agua labelled gas tankers that drove to specific marks throughout the city for maximum saturation. Although initially succeeding in his plan, the dispersal operation was soon halted by the BSAA with assistance of their elite Silver Dagger counter terrorism unit and the DSO. Glenn Arias was killed shortly after he mutated into a monstrosity reminiscent of the Tyrant B.O.W.s atop his secret hideout, preventing any further planned outbreaks of the A-Virus. The BSAA then quickly dispersed the Vaccine strain over New York City via helicopters before presumably issuing proper vaccinations to affected areas across the world.
With a vaccine developed to combat the A-Virus infection, it was rendered useless as a bioweapon on the blackmarket as stated by Rebecca Chambers.
The A-Virus strains are regarded as Group VI retroviruses in the Baltimore classification, and replicate by inserting positive-sense single-strand RNA into a cell along with the enzymes Reverse Transcriptase and Retroviral Intergrase. The RT enzyme, not found in animal cells, will then transcribe negative-sense single-strand DNA using the invasive RNA as a template. Using the cell's own RNA replicase, the viral DNA strand will create its positive-sense counterpart, resulting in double-stranded DNA. The RI will then break down the "3 prime" end of the strand, as well as the cell's own DNA so they can be attached. The result is a cell with mutant DNA. The cell is now programmed to create the original viral RNA as well as the necessary enzymes and the capsid protein to protect this genetic material.
As Type I A-Virus is known for its delayed effects, it is likely that it works via the exploitation of the lysogenic cycle. In this cycle, mutant DNA will replicate daughter cells as normal. These cells will have viral DNA in their genome, but will not produce the virus. This cycle will continue until the mutant cells switch to the lytic cycle, wherein the virus is produced, destroying the host cell in the process.
The triggering of the lytic cycle is brought on by chemical reactions in the body, which is achieved by a Type II infection. This virus was able to spread as an airborne pathogen, and was to trigger a reaction only in people infected with Type I. While this was the intended means of triggering viral activation, it was known to happen spontaneously, as was the cause of the Great Lakes killings.
The third strain is Type III, engineered as a means of preventing or reversing the violent and cannibalistic impulses typical to Zombies. As Type I and Type III both insert Plaga DNA into the human genome, Zombies will ignore Type III mutants.
A vaccine to the A-Virus was developed at the Alexander University Institute by Dr. Rebecca Chambers with assistance of her fellow researcher Aaron. This vaccine was only a prototype at the time and, though Chambers showed signs of resistance to the virus, she suffered a relapse soon after.