(Capcom's primary storyline)
"The Abyss" is a retrovirus discovered in the early 2000s. The virus spreads through fluid transmission, and is estimated at having a 99.76% chance of infection.
This virus was isolated in the body of a fish native to the Kermadec Trench east of New Zealand. Its discoverers, an expedition team from the Montpellier University of Marine Science, noted that the virus altered animal bodies to break down fat, improve muscular strength as well as developing a skeleton more capable of surviving in high pressure environments.
Following its discovery, Montpellier worked alongside the FBC in genetically engineering a viral weapon by splicing the Abyss' genetic material into that of the t-Virus.