The other answer has covered resurgence from natural reservoirs. I'll cover man-made resurgence. There are copies of it in all major countries' virology labs.

The other answer has covered resurgence from natural reservoirs. I'll cover man-made resurgence. There are copies of it in all major countries' virology labs. Furthermore, it can be synthetised from scratch since 2002, see NYT. NYT: https://www.nytimes.com/2002/07/12/us/traces-of-terror-the-science-scientists-create-a-live-polio-virus.html https://www.nytimes.com/2002/07/12/us/traces-of-terror-the-s...