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

> Construction of an infectious horsepox virus vaccine from chemically synthesized DNA fragments

https://journals.plos.org/plosone/article?id=10.1371/journal...

In theory, it's very much doable. We brought back an extinct cowpox virus a while ago using mail-order DNA. Did you know that Smallpox's nucleotide sequence is freely available online?