How is it that we haven’t seen more disasters caused by missing suitcase nukes or smallpox? My guess is the security services are better at keeping things stable than we think but it also probably means you’re under perpetual surveillance.
Smallpox on the other hand is much more likely to just get your home country nuked into oblivion if they find who deployed it. It's effectiveness as a weapon is going to be limited in first world countries with ring vaccination programs and epidemiologists. On the other hand it would be devastating to the countries that would have a ideological reason to deploy them.
Modern terrorism is a game of chicken. You want to be successful enough to degrade the enemy without triggering a backlash. Smallpox and nuclear weapons are great deterrents, however deploying them would mean the end of the countries and organizations using them.
Dr. Strangelove: "Of course, the whole point of a Doomsday Machine is lost, if you keep it a secret! Why didn't you tell the world, EH?"
I think it's just that nobody has both the ability and desire to destroy the world at the same time. Do we even know if smallpox would kill a modern human?
The last person to die of smallpox died in my lifetime. I hope I'm not so old that I'm no longer considered a "modern human".
Plus, effective heard immunity requires something north of 85% of the population. Smallpox vaccination rates are not even 50%.