I dont believe there is Biological attack on China coming from USA. This is very risky action since if the attacker can be found, there will be retaliation from China into USA. Just like no body want to nuke any body, particularly if the victim and attacker has similar capabilities.
The disease outbreak have been going on since human history ever recorded even our prophet has given an advice if such out break comes.
I also dont think US leadership is such low in morality that make them making such attack.