not only that, people are explicitly trying to bring back measles, and succeeding at it

If we gave every living human on the planet the polio vaccine as soon as they were able to receive it, it would still exist in the wild somewhere.

If new generations are not also given the vaccine, it will eventually return.

Probably the same with HIV, although I could see it being somewhat more difficult for that to make a resurgence. If we inoculated the entire human race against it, it could possibly eradicate HIV for several generations, if not permanently.

Humans are the only reservoir for polio, so that’s not true. We could theoretically wipe out polio just like smallpox because there are no animal reservoirs.

Huh. TIL. That boggles the mind though. Makes me wonder where it came from, or if perhaps its source was somehow killed off or otherwise destroyed, or if perhaps there is a reservoir somewhere that we simply are not aware of?

There’s a fair amount of viruses that have no animal reservoir.

Either they evolved from a different human virus, or from an animal virus.

Measles for example evolved from a livestock virus called rinderpest and lost the ability to infect cattle as it became a specialized human virus.