The G-Virus is the driving factor of the events of Resident Evil 2. The only known successful host of the virus is the character Dr. William Birkin, who dies at the end of the game.
According to the series timeline, it was developed by Dr. William Birkin after its basis was discovered in the mutated body of Lisa Trevor. She had displayed an unexpected immunity to the NE-α parasite that they tested on her, and Birkin and his associates found what would become the G-Virus as the cause of this immunity. It turned out that the G-Virus was a mutation of the Progenitor virus she had originally been infected that arose by absorbing all the other mutagens she had been implanted with. Dr. Birkin received approval from Umbrella to begin studying the virus, and completed his work on it in 1998. However, due to disagreements with his superiors, he decided to keep the research to himself and made a deal with the U.S. military to extract him. In response, Umbrella sent their Special Forces Unit to steal his research and virus samples. The unit shot Birkin and left him for dead, but while dying, he injected himself with a sample of the virus, and became the creature known as G. He tracked the USFU team to the sewers, and during the ensuing onslaught, several T-Virus vials were dropped into the water, inciting the city-wide infection of the game. However, agent HUNK, the lone survivor of the USFU, was able to escape and deliver the virus to Umbrella.
The mutations induced by the G-Virus tend to be extremely volatile. Unlike the T-Virus, the mutations caused by the G-Virus occur exponentially faster, and continue to be induced so long as the host is active, even lacking external stimuli, though they are more rapid and substantial when the host is wounded. Also, the G-Virus possesses none of the necrotizing properties that the T-Virus is known for. Furthermore, the virus is transmitted through implantation or ingestion, rather than physical contact or injury. It can only be accepted fully by hosts with a suitable genetic makeup, specifically a blood relative, so during the game, G seeks his daughter Sherry Birkin in order to successfully reproduce. However, it can also partially infect others by implantation of G-embryos, producing severe mutations that culminate in the host being able to release further G-embryos. Hosts, called G-mutants in the game, create their offspring by orally implanting small, parasitic organisms into a living host through the palm of their hand. If the host is compatible, the embryo begin to pupate and assimilate the host's body, turning them into a mutant of immense strength. If the host is incompatible, the embryo will rapidly grow inside the host's body, and will burst from their chest in a matter of minutes. They will then quickly mutate into their adult form and search for another host.
Though the G-Virus is a somewhat useful tool for trans-humanism purposes, it is inefficient as a bio-weapon, due to its poor communicability. However, it is more aggressively contagious compared to the T-Virus, which does not actively seek to infect others.
The G-Virus can be cured by DEVIL, an experimental vaccine. However, the vaccine is only effective during the initial stages of an impregnation-induced infection, before the embryos gestate.
