The G-virus is an incredibly potent mutagenic virus from the Resident Evil video game series. First appearing in Resident Evil 2, this substance was both powerful and highly unstable. The mutations it triggered in life-forms such as humans proved to be excessively violent and totally uncontrollable. The G initial means "god".
The G-virus was first discovered by Dr. William Birkin during experimentation on Lisa Trevor in Umbrella's Arklay Research Laboratory. It was first produced as a by-product within Lisa's body, which had been exposed to multiple viral strains during her incarceration. As it turned out, the G strain possessed fierce regenerative qualities and would allow those infected to rapidly heal and grow stronger, essentially granting its hosts biological immortality.
Birkin conducted extensive research on the G-virus at Umbrella's underground lab complex beneath Raccoon City and completed his work in September 1998. He had originally intended to use the virus to enhance his position with Umbrella, but he began to disagree with the motives of his superiors and decided to keep the research to himself. He attempted to contact the US military so that they could retrieve him and pass on his research to the government, but Umbrella intercepted Birkin before he could be retrieved and attempted to seize a virus sample. Birkin was gunned down by Umbrella troops who took an attaché case containing several vials of the T and G viruses. Birkin managed to save himself by holding on to one vial of G, which he injected himself with and underwent a rapid and horrifying transformation.
Upon his transformation, Birkin's powers of thought and reason quickly deteriorated, and he became driven by the virus itself and sought to infect other potential hosts. He produced parasitic embryos within his mutating body and attempted to implant them in others. His virus-driven rampage ultimately resulted in the accidental release of the T-virus which spread throughout Raccoon City, dooming the town to destruction.
The mutated Birkin had perished during the Raccoon Incident and it was hoped that the danger posed by the G-virus would have ended there, but Umbrella had successfully managed to retrieve Birkin's remains and were able to recover the virus from the remains. Ada Wong, a spy working for another unnamed organisation, had also managed to acquire a G sample. While the virus had not been fully eradicated, research on it by various organisations was kept limited due to the virus' volatile nature.
Very few G-virus mutations have occurred as only a handful of individuals have ever been exposed to it. First-generation infectees - simply dubbed as "G" - undergo rapid and horrific mutations that seem to defy all laws of biology, their forms regularly shifting and altering in impossible ways, forming new organs and appendages over time or in reaction to injury. These individuals also asexually produce parasitic offspring that seek out new hosts to infect.