(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/mutated or killed. In 2012, the potent anti-virus was released, beginning the resurrection of the Earth and destroying anything infected with the T-Virus.
The T-Virus, which had been created to help the sick and dying, was revealed to have a rather nasty side-effect once people die they would reanimate as Undead that sought to eat the living. Umbrella co-founder Dr. Isaacs saw such a result as profitable, killing his business partner James Marcus and taking control of the corporation until James' daughter Alicia was old enough to take control of her 50% of the company.
Years passed and Isaacs called a meeting with the High Command, explaining that he had done extensive research about the world. Humanity was slowly killing itself and the planet. Overpopulation, potential wars, limited resources, illnesses without cures, global warming, etc. Using "Noah's ark" as inspiration, Isaacs proposed they release the T-Virus to kill everyone who didn't work for the Umbrella Corporation. While the "cleansing" occurred, they would wait out the events safely underground. Once the rest of humanity was gone, they would return to the surface and restore the world to their specifications.
Though Alicia was against the idea, the rest of the board voted in favor of Isaacs plan. Hoping to stop the plan, Alicia uploaded a recording of the meeting into the Red Queen, which was programmed to obey and protect Umbrella employees, but to also value human life. As a result, the Red Queen was unable to directly prevent the plan; she did the next best thing. When it came time to create clones to protect the secret Hive entrance at the Looking Glass House, the Red Queen selected a clone of Alicia and programmed a strong conscience to stop Umbrella Corporation into it, creating Alice.
The First Outbreak
Alice was tasked with protecting the mansion fellow security guard Spence. Per her programming, Alice tried to make the illegal experiments conducted by the Umbrella Corporation public, by helping an undercover environmentalist named Lisa Addison acquire the necessary evidence to bring down Umbrella. However, Spence decided to steal the T-Virus and Anti-Virus, to sell on the black market; he released the virus into the Hive, forcing the Red Queen to try preventing the outbreak Isaacs wanted. As result, she killed all the Hive employees and released a nerve gas to stop Spence. The deceased were sealed into the rooms they died in, preventing the now-undead from escaping. Umbrella sent a commando team in to find out what happened, breaking into the Hive and dragging Spence, Alice and Lisa's brother Matt with them. The Red Queen attempted to stop the team, killing some; however, they refused to listen to her warnings. They shut her down, causing all the infected to be released. Everyone except Alice and Matt died before the Hive could be sealed again. However, Matt was scratched by a Licker, which caused him to mutate. More Umbrella troops arrived, taking both captive for experiments.
Alice was infected with the T-Virus, bonding with it a cellular level and didn't mutate; this was likely thanks to her original's progeria, which the virus couldn't affect long. Umbrella reopened the Hive, allowing the Hive outbreak to evolve into the Raccoon City Incident. Umbrella decided to see if Alice or the mutated Matt, now called Nemesis, would be the better version of successful T-Virus assimilation; both refused to kill the other and turned on Umbrella. Seeing both a lost cause and that the infection would soon become airborne outside Raccoon, as it had been 3 days since the outbreak, Umbrella launched a nuclear missile at Raccoon City Hall, destroying the building and much of the city.
Umbrella recovered Alice's body from the wreckage of a helicopter, deciding to try bringing her back to life. They also squashed the news footage of the outbreak, labeling both Jill Valentine and Carlos Olivera as terrorists trying to blame Umbrella for an "accidental" meltdown at the power plant. Alice was successfully brought back to life, with enhanced physical abilities and psionic powers. She managed to escape with the help of her friends, but Umbrella would be able to track the new Project Alice through satellite feed. With data on Alice, Nemesis, the undead and the mutations from Raccoon City, Umbrella began feverishly working towards the decimation of the world's population.
Initiating the Plan
To better help them understand the nature of the infected, Umbrella had reactivated the Red Queen and charged her with using the Umbrella Prime testing facility to conduct the research. Using simulations in major cities of the world at the facility, Umbrella shows the potential the T-Virus possessed to the world and sold the countries the virus, building a new arms race in the hopes that the virus would be stored poorly and begin the infection. "Patient zeroes" (people infected with the T-Virus) were also released into various countries by Umbrella to ensure the initiation of outbreaks around the world.
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. 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. To pacify the staff that would be forced to hideout in the underground Umbrella facilities, a clone of Dr. Isaacs was created for the purpose of trying to domesticate the undead and make them subservient.
The World goes to HellHumanity entered a dark age as the world was overrun by the undead; though stupid, zombies were deadly due to their numbers. As shown in Afterlife, as the number of infected grew, prisoners were released to help fight back. However, the virus had an additional, unseen, side-effect; it didn't just bring the dead back to life, it also affected the Earth itself. Rivers and lakes dried up, forests became deserts and meadows became wastelands. The Earth began to wither and die from Umbrella's actions.
What few survivors that were left learned to stay on the move and avoid large cities where the infection was at its worst. 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 consciences.
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".
In the decade that the outbreak occurred, things grew progressively worse. The undead began mutating, growing mandibles for catching prey. The zombies also started getting smarter, rather than pointlessly wander or wait outside the strongholds of humanity, they instead began burrowing. Worse, the wild life mutated as well, created giant winged monstrosities that made air superiority void.
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. A brainwashed Jill attacked Arcardia with Umbrella, killing survivors on whim. Wesker pretended to defect from Umbrella, gathering the remains of the U.S. army together with his troops at the white house, along with Leon Kennedy's group. This was all part of his mission to wipe them out.
Ending the Pandemic
Following the betrayal of humanity by Wesker at the White House, humankind is reduced to just over 4,000 people left, gathered in only a few settlements based around the world. This meant 1,729,548 people died between Extinction and The Final Chapter. One such settlement was 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 while other Undead armies led by Umbrella forces went after the remaining settlements in other parts of the globe. 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 Umbrella 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.