(Covers information from Anderson's universe)
The Global T-Virus pandemic was a global-scale outbreak of the T-Virus, triggered by two separate outbreaks in Raccoon City and Tokyo, Japan in 2002. By 2005, almost the entire human population had been either infected or killed. In 2012, the potent anti-virus was released, beginning the resurrection of the Earth and destroying anything infected with the T-Virus.
Following the Raccoon City outbreak, Umbrella co-founder Dr. Alexander Isaacs saw a chance to make use of the T-Virus for "saving" the world's resources from humanity's overpopulation. Getting co-owner Alicia Marcus to back him up, Isaacs convinced the board to agree with his plan. The Umbrella High Command would be put into cryo-stasis in the Hive, along with their families, while the T-Virus was released to infected the rest of the world and "cleanse" it of the unworthy.
To help them understand the nature of the infected, Umbrella had the Red Queen reactivated and charged with using the Umbrella Prime testing facility to conduct the research. Using simulations of outbreaks in major cities, the Umbrella Coroporation built a new arms race, sealing the T-Virus to countires around the world. "Patient Zeros" (people infected with the T-Virus) were also released into a country by Umbrella, to ensure the intiation of outbreaks around the world. To lure survivors into their clutches so they could be experimented on, Umbrella prepared a false safe haven called Arcadia which was a tanker hiding a testing facility within.
Since Isaacs and Alicia Marcus would be placed in cryo-stasis with the rest of Umbrella High Command, Albert Wesker was made chairman of the Umbrella Corporation during the outbreak. Having complete control of the organization, Wesker operated from the main facility under Shibuya Square in Japan. A clone of Dr. Isaacs was placed in charge of researching Project Alice, in the hopes that the undead could be domesticated. Should a clone fail to do so or become insane, it would be killed and replaced.
Humanity entered a dark age as the world was overrun by the undead. As shown in Afterlife, as the number of infected grew, prisoners were released to help fight back. The T-Virus didn't just bring the dead back to life, it affected the Earth's ecosystem; lakes and rivers dried up, forests became deserts and life on Earth began to wither and die. What few survivors that were left learned to stay on the move and avoid large cities where the infection was at its worse. Speed and constant mobility were their only defense against the unending horde of undead. A convoy led by Claire Redfield and Carlos Olivera was one such example; however, they always ended up too late to help others that radioed for assistance, something that weighed heavily on their conscieneces.
Occasionally, the humans would be aided by nature, as shown in Resident Evil: Extinction. Swarms of infected crows were known to feed on flesh, indiscriminately. The entire undead population of Las Vegas was purged in this manner. On the other end of the spectrum, there were survivors who remained in their cities, defending themselves vigorously, as shown in Resident Evil: Afterlife.
In Resident Evil: Retribution, the White House itself appears to have become another safe haven during the outbreak; surrounded by large walls and other barricades, which prevents millions of zombies from getting in. It may possibly house some remnants of the Military, and Leon Kennedy's "resistance".
The spread of the t-Virus was devastating, but it was not absolute. According to the White Queen's satellites scans, out of the planet's pre-outbreak population of 6.5 billion (US numbering system) human inhabitants, there were 1,733,548 people uninfected on the surface. In Resident Evil: Afterlife, survivors are discovered taking refuge in the LA jail and over 2,000 known survivors, including Alice, Claire Redfield, Chris Redfield and K-Mart are aboard the Arcadia, captured by Albert Wesker in an effort to keep his infection in check by feeding on their flesh. The number of survivors dropped during Afterlife due to the zombies, Wesker, the Red Queen, and in-fighting.
Ending the PandemicEdit
Following the betrayal of humanity by Wesker at the White House, humankind is reduced to just over 4,000 people left, gathered in a single settlement based in a tower in the ruins of Raccoon City. Led by a clone of Doctor Alexander Isaacs, three massive Undead armies, each led by Umbrella tanks, launch an assault to destroy this settlement and end humanity once and for all. To counter this, the Red Queen, in truth an innocent pawn in Wesker's plot to destroy the majority of the remaining humans in Washington, DC, contacts Alice who alone survived Wesker's betrayal and informs her of a potent anti-virus developed by Umbrella and kept within the Hive. This anti-virus was designed to be airborne and kill anything infected by the T-virus upon contact. By releasing the anti-virus, Alice could end the pandemic and save what is left of humanity.
Alice travels to the ruins of Raccoon City where she helps the survivors repel the assault of Isaacs and the first Undead army. Accompanied by a team of survivors, including Claire Redfield, Alice enters the Hive where she encounters the real Isaacs and Alicia Marcus, daughter of Doctor James Marcus who created the T-virus in the first place and of whom Alice herself is a clone. With the aid of Alicia and the Red Queen, Alice defeats Wesker and chases Isaacs, apparently killing him. Alice takes the anti-virus from Isaacs and attempts to release it upon the surface, only to be stopped by Isaacs who is killed by his clone. Alice releases the anti-virus, killing the thousands of Undead in the vicinity. The Hive is also destroyed by Alice with explosives, killing Wesker, Alicia and the thousands of Umbrella employees and High Command held within, effectively destroying the Umbrella Corporation.
While the anti-virus is released into the world and is airborne, it has to travel on the winds and will likely take several years to circle the globe and kill everything infected by the T-virus and ending the pandemic.