According to the series timeline, after the Ebola virus was discovered in Africa, Umbrella decided that it would make an efficient bio-weapon due to its near 90% death rate. It obtained samples of the virus, circumventing the fictional Biological Weapons Prohibition Pact by claiming that they were researching it in order to find a cure. However, the virus proved unsuitable for their uses, as it cannot survive for long outside of a human host, and is extremely vulnerable to UV radiation. Furthermore, it kills its host rapidly, and is only contagious through direct contact, severely limiting its ability to spread. Umbrella decided to create a new virus based around the idea of a heavily infected host that could still move around and actively infect others.
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 Tyrant virus, or T-Virus for short, is the main virus used by Umbrella in the series, and is responsible for the creation of most their BOWs(Bio Organic Weapon).
According to reports in the game, Dr. Marcus, Albert Wesker and Dr. William Birkin succeeded in creating the Tyrant virus at the Arklay Labs on December 4, 1978 through synthesis of the Progenitor-Ebola virus and leech DNA. Through this synthesis, the photosensitivity of the early Progenitor-Ebola strain was replaced with pyrosensitivity, a property that can be seen in most of the series's enemies.
The Tyrant virus supposedly operates similarly to most other viruses, but also has the abilities to animate dead tissue, to substantially mutate its host, and to infect nearly any tissue in any type of host. It animates dead tissue by killing and replacing any mitochondria in infected cells, and then combining with these cells to produce enough energy for motor and lower brain functions. By doing this, most of the bodies systems, such as the circulatory or respiratory systems, are made redundant.
However, this process has the drawback of severe necrosis in the host, and produces the distinctive rotted appearance of most BOWs. The mutations are produced when the virus incorporates itself into the host's RNA and considerably alters it. Creatures with genetic structures different than humans generally show less severe mutations, and usually only increase in size.
As a side effect of the virus's cannibalization of its host, specifically its digestion of the host's frontal lobes, all hosts suffer from greatly increased aggressiveness. In previously living hosts, the virus also damages the hypothalamus, which results in a flood of neurotransmitters, enzymes, and hormones which induce a psychotic rage and hunger in the host.