(Covers information from the S.D. Perry novels)
The T-virus was a mutagenic virus engineered by the Umbrella Corporation. It was engineered by Drs. Helen Black and James Trenton Darius. It was used as a marketable bio-weapon. It is known to be able to infect people as an airborne virus at at least some point in its lifetime.
The T-Virus was developed by researchers James Darius and his wife Helen Black. It was the end-product in their genetic research, being intent on developing a virus which could promote cellular regeneration. Umbrella, their employers, considered such a treatment to be harmful to their financial empire and had them killed in a fire. Dr. William Birkin, an employee of theirs with a more appropriate word view, became familiar with their notes and drove the project into bio-weapons development instead.