The GENE virus (G-virus) is capable of creating a superior Bio-weapon to Umbrella's t-virus. An individual infected with the virus would transform into a hideous monster determined to reproduce. They reproduce by implanting embryos in their victims body. If the Host's blood type is not the same as the carriers, then the embryo will eject itself from the body via the chest. The creature will then grow quickly into a more-hideous monster.
If the host is a genetic match, then the embryo fuses with the host to become a new and superior creature. What that is, however, is not yet known but is a theory is that a compatable host would become similar to Birkin.