Comment by tialaramex

It might be possible to reintroduce Smallpox and I guess that idiots who also think coal is a good idea might actually be stupid enough to make it happen. But, fortunately humans did wipe out one other disease and unlike Smallpox it wasn't deemed useful as a biological weapon so AFAIK nobody kept copies, it's just gone.

https://en.wikipedia.org/wiki/Rinderpest