(Covers information from Anderson's universe)
The anti-virus is the only compound capable of purging the t-virus. It was created by Dr. Charles Ashford as a means to help his daughter, Angela Ashford, to retain her mobility and keep the t-virus mutation in her system in check.
In its liquid state, the anti-virus' visual cue is green as opposed to blue like its counterpart. While it is known to cure t-virus related infection, the anti-virus is not a fool-proof antigen. Rain Ocampo, who had been bitten several times and spent over an hour in an infected state, died and reanimated as an Undead a few minutes after being administered the anti-virus. Dr. Sam Isaacs was bitten and infected by a Super Undead, and several massive doses of the anti-virus could not save him from extensive mutation. It was even implied by Slater that the anti-virus could mutate Issacs; this is possible as the anti-virus, despite being a cure, can be dangerous if taken in massive doses similar to real-world medicines like paracetamol.
However, it did work for Carlos Olivera three hours after he was bitten by his infected partner, Yuri Loginova. It is likely that the dosage of the t-virus in Rain's body was significantly more than in Carlos' as a result of being bitten more than once, thereby increasing the overall dosage and giving her a smaller time frame in which to safely inject the anti-virus without mutation. However, when Carlos was infected a second time, even though there was only a few hours between infection and arriving at the North American Research Facility, Carlos believed that his infection was too far long to be cured and didn't even attempt it.
In Resident Evil: Extinction, the White Queen explained that because of Alice's unique situation as the only human to bond with the T-virus, her blood contained a possible cure or anti-virus to the t-virus, which had since swept across the globe. Umbrella's North American Research Facility contained all of the equipment that she needed to create another cure, but whether or not Alice succeeded or attempted the endeavor is never revealed.