So.. just imagine you want to take over the world. Nukes won't cut it. They lack deniability and people have defences against them, and of course they'll nuke you back. So how about biological warfare. You've got a virus. Now the thing is you want it to only take out your enemies rather than yourself. How would you go about engineering that? It's tricky. You'd need to have it target particular races. Now China is ethnically pretty homogenous, moreso than any of the other major players, so it can just go with "everything but Han Chinese please" - maybe a milder impact on their own and a harsher impact on the rest. Alternatively, they may wish to keep Africa running as they have been working on turning Africa into cheap labour for their middle classes to exploit so maybe just white people.
Thing is, even if it doesn't kill us off, just keep releasing in steady streams and it'll debilitate us so much through effort in fighting it, it'll put so much friction in the economy as to kill it. It's similar to the cyberwarfare carried out by Russia and China. Sure they'd like some secrets if they can get them but the constant swarming of any server that goes online isn't just about what they can get, it's about making the effort of securing stuff so onerous as to make Western businesses uncompetitive due to the extra friction. So we in the West will have to put enormous effort into keeping people healthy, and eventually our economies get crunched into oblivion.
In short, nuke China.