The T-Veronica Virus (also once referred to as the "Veronica Virus") is a variant of the Progenitor Virus created by Alexia Ashford which, under the correct conditions, allows the host to retain full intellectual capabilities. It is named after Veronica Ashford, the Ashford family's very own matriarch.
The basis of T-Veronica research, came with the discovery of a retrovirus existing within the genome of some ant queens. Alexia mistook the socialist nature of an ant colony for one with complete adoration for their "queen", and likened it to her own megalomaniac lifestyle personality. She then began considering the possibility of merging the genome of this virus with that of the Progenitor virus. Plant genes were also added into the virus' makeup.
The first step in human testing came in 1982, when Alexia and Alfred infected their "father", Alexander Ashford, with the virus. For whatever reason, he failed as an adequate experiment; he was imprisoned within the Antarctic Base, terrifying the facility's workers with his inhuman growls, earning him the name "Nosferatu". Soon after her father's degenerative mutation, she christened the virus "t-Veronica", in name of her cell-donor.
It was soon after this that Alexia decided to experiment on herself. In order to prevent the degeneration Alexander faced, Alexia concluded that going into a cryogenic coma for some fifteen years should allow her to adapt to the virus.
At one point in this year, Albert Wesker intercepted a rumour regarding the existence of this virus. Expecting it to be somewhere in Alfred's Rockfort Island base, he and the Hive/Host Capture Force attacked the island, leading to a biological outbreak of the T-Virus. Eventually, it was discovered by Wesker that Alexia was in fact alive at the Antarctic Base; he concluded, rightly, that he would find the virus there. He left via a submarine on his way to the base, situated in the Ross Ice Shelf.
Shortly after Alexia's revival, she proceeded to experiment on Steve Burnside. While he didn't have the benefit of a fifteen-year period of hibernation, Steve managed to counter the virus not long into his mutations, as he regained his sanity - unlike Alexander, the virus had not killed off his brain cells; he died soon after when a psychically-controlled tentacle smashed him against a wall.
Alexia herself was killed by Chris Redfield with the experimental "Linear Launcher". With nothing surviving the ensuing destabilisation and explosion of her body, the capture force instead took Steve's body for experimentation.
Carla Radames acquired a sample of the t-Veronica virus and experimented with it, removing the genetic code of the virus which created sudden genetic mutations, and then she combined it with parts of the Progenitor virus. She did this to ensure that there would be no apparent brain damage. She called this variant of the t-Veronica virus "T-02".
This way she got rid of the need to have the virus in your body for several years, either by cryogenic sleep, like Alexia Ashford, or by regular organ transplants, like Manuela Hidalgo. This increased the potency of the virus, because results came in moments instead of years.
Some years later, drug lord Javier Hidalgo ceased purchasing from the Umbrella Corporation, who had desperately begun to inflate their bio-weapons prices due to the Raccoon Trials. The t-Virus variant he had infected his wife, Hilda, with to treat her illness had begun to backfire. It was at this point that Wesker began doing business with him, selling him samples of t-Veronica at a much more reasonable price. The virus, when given to Hilda, unfortunately transformed her into a grotesque aquatic creature. However, Javier's faith in the virus had not diminished when he infected his daughter, Manuela, with the virus in the hopes that her illness would not degenerate her as bad as her mother. Javier later came to the conclusion that a means of countering the virus' control over the body would be for her to go under a series of organ transplants for fifteen years - this way, she would remain in control of her body and be able to live with her father. His crime syndicate began kidnapping young women of Manuela's age to be used in forced organ donation.
When US Government agent Leon S. Kennedy and USSOCOM member Jack Krauser began investigating the region in search for the Umbrella researcher who had been selling bio-weapons to Javier, they discovered Manuela. Being led to Javier's mansion, they discovered the organ donors and Hilda. Soon after, Javier infected himself with the virus by merging with the V-Complex in order to kill Leon and Krauser. His conciousness soon faded and the horrific creature took over; it would no longer resist trying to kill her. It was at this point that Manuela also realized the psychological aspect of t-Veronica: if she gave in, she would no longer feel pain; if she resisted, she would die human. She chose to die human in order to save Leon, throwing her blood at Javier until he eventually died. Although weakened from blood loss, she nonetheless survived as an ordinary human.
There is a link between mindset and mutation with Progenitor variants. This can be seen most-noticeably with the T-Veronica virus, where 2 people (Manuela and Steve) have been able to counter mutations by regaining control over their bodies.
A third person would be Manuela's mother, Hilda, who regained some part of her humanity at the sight of Manuela and was kept at peace when she sang a lullaby that Hilda sang to her as a child. Unfortunately, when she was too weak to sing the song, the non-human personality regained control and attacked Leon and Jack. Javier, like Alexia, was also able to control himself when mutated by the virus; however, he allowed it to take control, eventually defeating the human mind and assuming complete control as an uncontrollable beast. In order to perfectly co-exist with the t-Veronica virus, the host must live with it for a number of years.
Alexia Ashford achieved this by remaining in cryostasis for 15 years.