The Progenitor virus, also known as the Founder or Mother virus, is the first of the mutagenic viruses, and the basis for most of them.
According to the series timeline, it was discovered on December 4, 1967, by Dr. Edward Ashford, Dr. James Marcus, and Lord Ozwell E. Spencer, while they were studying the Ebola virus. Though Ashford wanted to use the virus' regenerative abilities to help the handicapped, Marcus and Spencer wanted to use it for the bio-weapons project, and after Ashford's death in 1968, they were able to begin their desired research.
It produces rapid and uncontrollable mutation in a host's genetic code, but the mutations were not coordinated enough to produce effective BOWs. In hosts with a genetic structure less complex than humans, mutations are less pronounced, and usually restricted to increase in size and aggressiveness. In order to enhance the virus's mutagenic properties, Umbrella created a variant of it through synthesizing it with the Ebola virus, but this strain retained Ebola's photosensitivity.
The Progenitor virus was originally going to be used in Resident Evil 4 in the fog version of the game, but was instead replaced by Las Plagas.
