Good question. That was my first thought, too.
1918 Spanish Flu Pandemic wasn't biowarfare, Legionnaire's disease wasn't biowarfare, Black plague wasn't biowarfare.....
However, from now on it will be completely impossible to convince anyone that WANTS to believe a newly emergent contagious disease is biowarfare, that it isn't. So I'm not even going to try.
People absolutely hate the idea of anything significant or bad happening in the world without it having a specific human cause; they HATE randomness or the appearance of randomness more than anything.
China is a classic area for a naturally occuring contagious disease to emerge or jump species and people were worried about that happening naturally long before people were really concerned about biowarfare.