One of the major plot elements of the Resident Evil series is the outbreak of various virus. These viruses are the creation of the fictional Umbrella Corporation, which is attempting to use them to create Bio Organic Weapons (B.O.W.s).
According to the series timeline, Umbrella Corporation founders Ozwell E. Spencer, Dr. Edward Ashford, and Dr. James Marcus began looking into viral mutation sometime in the middle of the twentieth century with the intent on finding a way to create the perfect biological weapon. This would involve finding or creating a viral agent that could not only be controlled, but could be used to create new bio-weapons by infecting a host organism and mutating it into a powerful creature that could act as a soldier and follow orders, as well as spread the virus that created it into enemy territory. On December 4, 1967, the three men succeeded, with the discovery of the Progenitor virus and from this came the birth of the Umbrella Corporation.
Read more about G-virus at Wikipedia ...
Official Site: Capcom
Links: Wikipedia Resident Evil Fan: A New Blood Resident Evil Official Website