The T-Veronica virus is the main virus used in Resident Evil: Code Veronica, and is responsible for the mutation of several of the enemies in the game. There is also a variant strain of the virus named T-Alexia that is a diluted form of the virus created during its period of cryogenesis in its main host, Alexia Ashford, who is also the main antagonist of the game.
According to the series timeline, the T-Veronica virus was created by Alexia Ashford through synthesis of the Progenitor virus and the gene of a queen ant that contained an "ancient virus". She administered it to her father, but because the infection was not controlled, he became unstable and mindless. This led her to discover that the mutations could be controlled if slowly absorbed during cryogenesis. It is also worth noting that no in-game B.O.W infected with the T-Veronica virus shared the pyrosensitivity of the T-Virus; whether it shares the photosensitivity of the Progenitor virus is never shown.
Like the Progenitor virus, the T-Veronica virus causes rapid mutation, but the infection can be controlled and intelligence can be retained if the virus is absorbed over a long period of time — Alexia Ashford achieved this by remaining in a cryogenic sleep for fifteen years while the virus merged with her at a cellular level. Among the mutations to Alexia caused by the virus are the ability to exert control over the hive mind of the ants the virus was extracted from, combustible blood and vomit, and multiple tentacle-like muscles. It is suggested within the game that Alexia can control her mutations, though they become more severe after she sustains considerable damage in the player's final boss fight with her.
Though the T-Veronica virus is a useful tool for transhumanism purposes, it is useless as a bio-weapon, as its infection rate is low and it can only be transferred through direct injection.
