Its a solid myth that smallpox would be some sort of movie style 'outbreak' disease.
That bad for some individuals? Absolutely.
That bad for everyone? Not a chance.
The 'herd' immunity is alive and well, and will be for centuries, even with smallpox not running around. (Your mother never had to catch smallpox for you to have some natural resistance, but your 'bloodline' sure as hell needed exposure. Having said that, such immunity will go away eventually....).
Keep in mind, the 30% fatality rate you hear about INCLUDES the elderly, those already in poor health, etc. It also assumes EARLY 1900s medicine (which admittedly then AND now is just supportive care, but our supportive care is better).
And while the smallpox vaccination IS a vaccination that is harsher than average, it has the HUGE HUGE virtue of being able to give AFTER the disease is contracted (but before serious symptoms) and give some increased odds for survival.
Also, old vaccinations people got decades ago help your whole life, even without boosters. The boosters make you nearly immune, but if you were EVER vaccinated vs smallpox, you are MUCH better off than someone that was not. (Fatality for elderly people vaccinated when young, with no boosters, was less than 10%)
And if smallpox was re-introduced to the environment, we could have 99% of the country vaccinated in a few weeks, tops.
I'd be really really really surprised if accidental re-introduction killed more than few 100 in the initial release. ("Attck" re-introduction could be worse, but would still not be anything like 'out-break').
Having said that, re-introduction would almost certainly kill millions world wide before it got shoved back into a bottle. And every damn terror group out there would collect samples off the sick and dying.
And that 30% fatality number is NOT for the population, it is for the infected. So even if that number holds true, the population fatality rate would be a LOT less.
Vastly worse than anything we've ever seen? Yes.
But still not 'outbreak'.