There's nothing like bioweapons to level a playing field. The only problem with using the flu as a leverage tool is the flu tends to mutate quickly, as noted above, so it could blow back on the Chinese. Infectious diseases concern me the most of potential threats, because it's hard to defend against invisible invaders. One small lapse could be potentially fatal. I have a neighbor who died in his early 50's from the flu, and knew someone else who was a healthy, athletic male in his mid 40's, who died from it as well. Scary stuff!