(Capcom's primary storyline)
Arias developed the A-Virus in association of the Los Iluminados cult as part of his plan to bring about human extinction to atone for its sins. It exists as three strains: Latent, Trigger and Vaccine.
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 Animality Virus refers to three individual viral strains that work in tandem as a biological weapon: the Latent and Trigger Viruses as well as the Vaccine strain.
The Latent Virus is waterborne and transfers from host to host via fluidic contact such as through bites. This strain is naturally dormant, allowing it by design to infect a wide area before detection. Although it could mutate under unknown circumstances, the possibility of this was considerably sparse.
The second strain, the Trigger Virus, is described as causing the Latent strain to activate and zombify its host when the two are in contact. This strain is airborne, and was intended to be stored in gas canisters to be sprayed over a large area. The airborne strain is described as having a similar flash-point to gasoline, making it a highly flammable gas.
The Vaccine strain is notable in that it can reverse the effects of the Trigger virus and un-zombify people. In addition to its cure-like capabilities, the Vaccine strain was also developed so that zombies infected with A-Virus will not attack those vaccinated with this strain.
A true vaccine to the A-Virus was developed at the Alexander University Institute by Dr. Rebecca Chambers with assistance of her fellow researcher Aaron. This vaccination, although capable of eliminating all traces of the A-Virus from one's body, was shown to be unable to prevent later attacks from an A-Virus infected individual.