Best case : it was something intended to gently curtail the aging (and expensive to care for) Chinese population, but turned out a little too "successful" and ran out of control for a while, and spread outside the Chinese population. Mighty suspicious how they seem to have been able to "turn it off" in China now though, and compared to the rest of the world, end up with relatively few deaths now.Genuine question, why do you think they would manifacture a virus? What do you think their motive would be?
Worst case : a dry run (or even a warning shot) for out and out biological world warfare.