Most studies show brain damage.

If I wanted to be the central kingdom, with everyone sending me tribute, making everyone else dumber would probably accomplish that. And it'd be good payback for opium too.

I don't think this was a bioattack, but a more likely scenario is a leak of a bioweapon still being developed. I don't consider that even unlikely. If I assign that scenario, or one like it, even 10% odds, extreme precautions still make sense.