That's what this war is all about. Low tech bugs and germs have completely neutered our nuclear arsenal, as everyone knows that we won't use them.
If someone fired off a nuke, they would immediately be toast, the threat would end, and a million or so would be dead in an uninhabitable small area of the earth. Dire, but controllable.
If a madman reintroduced smallpox to the world, billions could die, and even if the madman was nuked, the smallpox would still be spreading. Human interaction would become very risky, and the world economy would grind to a halt, while the smallpox continued to spread. Dire, and uncontrollable.
The madman must be stopped!
Nah, we'd just vacinate everyone, which would take some time, but not all that long. The world did get on quite well before the vacine was invented you know. Well, maybe not "quite well", but it got along. The only reason we are vulnerable to smallpox now is that vacination worked so well in the past that the virus died out in wild for lack of vulnerable hosts.
Still, I'm hoping I've got some immunity left from my mid 80s military vacination for smallpox. (I checked my vacination card a couple of months ago, just to be sure when I'd had it)