Seeing as how smallpox has been eradicated, the appearance of even ONE case will let you know.
If the current anthrax situation is the precedent, then I simply can't share your confidence that the public authorities are going to be able to instantaneously diagnosis a disease that nobody has seen in 30 years, instaneously determine the full extent of the outbreak, and then instaneously notify the public.
The appearance of even ONE case of pneumatory anthrax in Florida should have let us know that there was an anthrax bioterrorism attack underway. However, even government authorities, as well as MOST of the people on FR, were in denial about this for almost two weeks. Not a precedent to inspire much confidence, I'm afraid.