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Prime universe (Capcom's primary storyline)
The ancient virus was a retrovirus which was responsible for a global pandemic.
Little is known of its precise history before modern times, but it was known to infect and mutate whales, which had developed a measure of resistance to the virus. A mutant whale was frozen in Alaska and preserved for three million years, partly perhaps as a result of its mutations. When a research team tried to examine the body, they were exposed to the virus and mutated into Zombies. The initial outbreak was contained when the US military began a Sterlization Operation in Alaska, but a sample later escaped from a USAMRIID facility and began spreading across the world.