The virus spreads like wildfire, but it seems to only be lethal in the elderly.
...so far.

I've got my doubts of course, but let's try a thought experiment. What if that is, in fact, true? What if it is as contagious as we fear, but only deadly to the old or infirm? That'd be an odd combination for what may be an engineered virus, but let's consider it.
In that case, it might not be a "weapon," per se, but a sick and sinister form of broom.
Let me explain.
This is 2020, the year Geopolitical forecaster George Friedman predicted that China would start to suffer an economic drain because of its burgeoning population of retirees.
And suddenly, a virus breaks out, spitting distance away from a Chinese facility, where an American company was researching strains of exactly this virus, which is effective at killing off the old but seems to leave anyone else.
And this happens at exactly the right time to guarantee that it spreads all over China in a matter of weeks.

How convenient for the Chinese government, don't you think?
But you might ask, "what if they get caught? Don't you think the population will rise up and revolt, outraged that their grandparents have been killed off by a bioweapon?"
Well, the fact that the Coronavirus patent was owned by an American company gives them plausible deniability. They can always simply claim "it wasn't us, it was the CIA." China has a long history of blaming the CIA for everything from the Dalai Lama's defection to the Tiananmen Uprising to the Hong Kong Protests. They'd believe it.

And what would that leave China with?

A population with no seniors to support, and a young and energetic population, all grateful to the CPC, and all with a burning grudge against the United States.