this is possibly boring to everyone but me, but a few people said that they were vaguely interested in this sort of post, so uhhh let's go for it?

humans have successfully eradicated one infectious disease that infected humans: smallpox.¹ we're also working toward eradication of several more (most notably polio). smallpox does continue to exist in labs (which is its own source of controversy), but the who is confident that it no longer exists in nature - basically, the only way future infections can occur will be a freak accident or bioterrorism. we don't need to vaccinate against them anymore² - they're gone. for once, we made the right thing go extinct!

that probably isn't a realistic goal for most infectious diseases, though. we can certainly eliminate them (either regionally or globally), but if we stop taking active steps to prevent their reintroduction (e.g., vaccination, sanitation systems), they'll come back. we can't wipe them off the face of the earth.

what's the difference?

every human disease that has either been successfully eradicated or is a target for global eradication has no other animal host. there's nowhere for it to hide when it's not infecting humans. that doesn't mean that they didn't originally spill over into humans from animals, but they can't jump back and forth. if you cut off its transmission in humans, you cut off its transmission.

one the other hand, if there are other animals that it can infect, we can catch it from interacting with or eating those animals. in some cases, those animals have clear signs of sickness - ebola has decimated many gorilla populations, for example - but in others, the disease just quietly lurks without causing any sickness.

it's the latter group that cause a particular problem when it comes to eradication, because these animals (called reservoir hosts) aren't really sick. that can make it very hard to identify the species at all (45 years on, we're still looking for the reservoir host(s) of ebola - we think they're bats, but hadn't been able to confirm that or the specific species last time i checked) or the communities that are carrying the disease once we've managed that.

so! that's why. this may or may not be interesting to other people, but yeah, that is why!

¹ we've also eradicated a disease that impacts livestock - rinderpest - but that's its own story.

² in theory. in practice, concerns about bioterrorism do mean that some people continue to be vaccinated against smallpox, even though the last naturally occurring case of it occurred in 1977. (the last case occurred in 1978 in the uk, after a lab accident led to someone getting infected and dying - following that, all stocks of smallpox were officially either destroyed or relocated to the cdc center in the usa or the vector institute in russia.)