The "G-virus", is the central plot point of Resident Evil 2. It is a highly advanced strain of the Progenitor virus with continuous mutagenic properties, but the only known successful hosts of the G-virus are Dr. William Birkin, his daughter Sherry, and Curtis Miller.
According to the series timeline, when Dr. William Birkin's test subject Lisa Trevor displayed unexpected resistance to the experimental NE-, he discovered that the cause was her infection with a primitive form of what he named the G-virus. Later, shortly after the events of Resident Evil 2, Agent HUNK and Ada Wong procured samples of the virus for Umbrella and Albert Wesker, respectively.
The G-virus is only transmitted through implantation or ingestion, rather than physical contact. It can only be fully accepted by hosts with a suitable genetic makeup, such as a blood relative, but can partially infect others by implantation of "G-embryos", producing severe mutations that culminate in the death of the host and the release of additional G-embryos. In general, the G-virus greatly accelerates cellular duplication and revitalization, at the cost of higher brain function. This causes the host to exhibit feral behavior, loss of moral reasoning, memory, and other higher drives, leaving only the urges for self-preservation and reproduction. The virus exhibits its symptoms within seconds, inducing extremely volatile mutations; a third eye is a typical outcome. The mutations accelerate when the host is wounded; this phenomenon caused William Birkin to suffer five increasingly gruesome mutations before expiring.
If the host is compatible, the embryo begins to pupate and assimilate the host's body, turning them into a mutant of immense strength. These hosts, called "G-mutants", create their offspring by orally implanting small, parasitic G-embryos into a living host through the palm of their hand. If the host is incompatible, the G-embryo will rapidly grow inside the host's body, and will burst from their chest in a matter of minutes. It will then quickly develop into an adult "G-creature" and search for another host. G-creatures periodically emit more G-embryos.
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. In the case of Sherry Birkin, DEVIL did not destroy the cells infected with the G-virus; it merely halted their spread, leaving her as a virus carrier.