(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.
Having seen the various results the T-Virus had during the events of Raccoon City, the Umbrella Corporation decided to profit off of it. Using the Umbrella Prime testing facility, they held simulations of viral outbreaks in major cities of the world; each was recorded, and shown to a country the city didn't belong to. Thus, Umbrella built a new arms race; however, selling the T-Virus severely backfired. The virus escaping confinement and infecting others at an uncontrollable rate.
One known instance is Tokyo, where a teenager spread the virus by attacking a businessman in public. Spreading quickly through the streets of Tokyo, every attempt at confining or suppressing the infection failed. Other countries suffered the same fate.
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. A journal discovered by a wandering Alice claims that a community in Alaska, dubbed "Arcadia", remained safe by virtue of its isolation. It would later be revealed to be a tanker under the control of Umbrella for the sole purpose of capturing survivors to experiment on them in order to learn how to perfect control of the T-Virus.
In Resident Evil: Retribution, the White House itself appears to have become another safe haven during the outbreak; surrounded by 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.
Through his experiments, Sam Isaacs came to the realization that 'Project Alice', who managed to evade the corporation for five years via staying off the grid and away from other survivors, was the key to their and humanity's survival. Her blood, being the only successful adaption with the t-Virus, could lead to a possible cure, granting total immunity to the pathogen's devastating effects. Upon learning this from the White Queen (the sister computer of the Red Queen), Alice herself was eager to help in the creation of a cure. Although, it is still unknown if she ever succeeded in creating a viable anti-virus, Alice remained steadfast in her vow to make Umbrella pay for their actions.