It's a little hard to shoot entire continents with .223 rounds. Should it turn out that America's very survival is mortally and immediately threatened by foreignors, then one possible defensive posture is to wipe out all life on all other continents, with very few exceptions made. It's not out of the question, and it is certainly technically doable with our own arsenals of nuclear devices, chemical weapons, and biological munitions.
Make no mistake, a biological attack on the U.S. would be a VERY serious development.
What if only six people die?
Yeah, the WTC would be like an appetizer. Rather than 5,000 dead, millions might be the new number. Bio/Chem agents are no laughing matter.