The G-Virus is the brainchild of Doctor William Birkin, created using advanced strains of countless viruses used in three decades' research of the test subject Lisa Trevor, whose body had absorbed and merged all the viruses that had been tested upon her, effectively rendering her immortal. Using the data and samples gleaned from Lisa, Birkin created his virus.
Birkin had planed to escape Lisa Trevor and Umbrella with his virus, hoping to join forces with his ally Albert Wesker in his quest to vanquish Umbrella; however, an Umbrella Assault Squad reached his inner sanctum in the sewers of Raccoon City and managed to kill him and steal a sample of the G-Virus. Birkin was left with no choice but to use his virus on himself as to kill the attack squad to prevent the theft of his virus. In the ensuing altercation, the valise containing G-Virus and T-Virus samples was destroyed, and rats consumed their contents, thus setting the scenario for Resident Evil 2.
Later, with samples possibly and probably provided from Umbrella agent HUNK, rogue agent Ada Wong, and William Birkin' G-Virus laden corpse, Curtis Miller was given a G-Virus sample to break into WilPharma and use it to cause a disaster at their central headquarters by mutating himself into a G-Monster and level the building. He used this sample to destroy WilPharma as a pawn in a ploy to ensure TriCell dominion of the bioweapon field. Later, a claw from the mutated Miller was recovered from the remains of WilPharma laboratories.
The G-Virus is different from the T-Virus in that it is not transmissible except in highly specific circumstances. It also involves a type of "forced evolution" that continually forces its host to improve itself upon the current circumstances; while this is similar to the T-Virus' capabilities, the G-Virus has a much faster time of adaptation, allowing hosts to transform in a matter of minutes, or given the right circumstances, seconds. This adaptation is continuous, and cannot be halted until the host's genetic makeup has suffered extensive damage, to the degree the virus cannot adequately counter it and starts random, faulty genetic mutation.
A carrier also tends to create G-Virus "offspring" that can infest living beings as parasites to fuel their explosive growth; if these latch into non-viable genetic carriers, the evolve into insane, cancerous anomalies known as G-Imagos. Also, the only viable way of transmitting the G-Virus is for the original carrier, which must have been infected with a viable sample of G-Virus, to implant one of the offspring into a viable genetic carrier (a blood relative of the original host). This will initiate a resequencing chain in the second generation host's body, adapting it more slowly to ensure the ravages of the G-Virus upon their systems.
Memories in G-Virus infectees are somewhat unclear. Most of the time, their animalistic nature is in control, but brief sparks of the people they used to be occasionally shine, especially in cases when confronted by people of their past capable of triggering memories. However, these sparks are brief and passing, and are to be considered the exception, not the rule.
An antigen, codenamed DEVIL, can be taken to counter the transformation of a second-generation host, but it is only viable to neutralize the G-Virus, not eradicate it.
Currently, a single living viable G-Virus source exists: Sherry Birkin, daughter of William Birkin. She is believed in custody of Albert Wesker or the U.S. Government.