Hyping the smallpox danger to Armageddon proportions in a book also causes it to sell well. Like we've never seen that before.
If anything, they were probably the "good old days" of smallpox.
Current smallpox would be modified to make it super hot. Preston quotes a number of USAMRID personel who feel that the soviets modified smallpox to make our vacine ineffective. According to the former head of their bio-weapon program, they typically tried to make pathogens triple-immunne to our treatments.
As for modern quarantine procedures, all of those procedures are modeled after natural outbreaks. The "ring vacination" method that led to wiping out smallpox doesn't work if you have humans causing outbreaks aimed at maximum damage. It is like trying to fight a forest fire with an invisible pyromanic running behind the lines starting more fires.
If you look at the mortaility rate, the amazing ease at which the virus spreads in the air, the "per-generation" growth rate it would have in modern society, the lack of a treatment, and the pure horror the mechanics of the disease inflict the only rational, well thought out response is that of grave concern.