As for plague, it must be remembered that plague is contracted fairly frequently in the United States, and has not produced a major epedemic.
Smallpox is another matter altogether, although there is the distinct possibility that much of the population still has a certain degree of immunity from past vaccinations, and the government is already preparing to vaccinate everyone.
With what? Their 12 million stockpiled doses? It'll be the rich, powerful, and military that get vaccinated first, then Doc's, police, and firemen. If you don't fall into any of those categories, don't plan on getting a vaccination. With a 30% rate of death associated with Smallpox, you have a pretty decent chance of living, but you'll be scarred for life. It will be at LEAST a year before production of new vaccine can begin, and probably two years until enough is made to get to everyone. One last litte scarry stat, there is an estimated 80% susceptibility in the population. The vaccination loses effectiveness in as little as 10 years.
Lets plug some variables into a nice little epidemiological equation:
Rp(average # of new cases per single case) = S(density of susceptibles in polulation) x B(tranmission rate [~1 for smallpox]) x L(average period of time host remain infectious)
What do we get . . . 16 news cases of smallpox for every single new cases. Of course there would be quarantines, and this rate would obiously slow down, but with a such dense population centers as we have in the US, the spread could be epidemic very quickly, especially since the differetional diagnosis is chicken pox or the flu.
Do NOT make light of a Smallpox attack. We are unprepared.