Civilization should do quite well if it ever got loose again and killed at it's normal rate ~ we'd end up with far more people than we had in the 1400s ~
There are even people with serious resistance to cholera, diptheria, typhoid, black death, and a number of other viral and bacterial diseases ~ there are also some downsides to that ~ But never mind.
“Smallpox is not a universally fatal disease “
Not when it has been engineered to be more lethal.