In the summer of 1977, an influenza epidemic began to spread through China; by fall, it was in Russia, and over the winter, it spread across the Northern Hemisphere. That was very normal, of course. Winter is flu season, and this was not a bad flu.
Everything seemed typical, until virologists got a chance to analyze the strain. Once they did, they were disturbed. Because what they were looking at was not the virus they would have expected to see. It was almost identical to a flu strain that had not been seen in the wild in more than 20 years.