Both smallpox and monkeypox are orthopoxes and there's some cross-over protection from smallpox inoculation (which at the beginning was probably cowpox, AKA vaccina, which is where we get the word "vaccination"). Protection certainly fades, but I find it interesting that this seems to be primarily something younger people get - and they grew up post smallpox eradication and so weren't vaccinated.

I'd also point out that people who have had chickenpox (another orthopox) continue to be immune into later age (though that might be an effect of lingering virus, as evidenced by shingles)

The original smallpox vaccine was relatively safe, balanced with the severity of the disease - but that small risk meant it made sense to stop vaccinating once smallpox was eradicated (possible because the reservoir was us humans).

There's a considerably safer smallpox vaccine available today. I'd get it if I was about to travel into one of the areas where monkeypox is endemic.

Given that rodents (not monkeys or humans) are the reservoir for monkeypox, I'd also wonder if it's been introduced into some new rodent populations. I don't think we have any kind of sentinel program that might spot that(?)