Polio is sooo close to being eradicated too, it's been a struggle for the reasons mentioned in this thread. Why can't they replicate how they eliminated Smallpox?
Ebola is not a real candidate for that of course because it has non-human vectors. But the disease probably would have never spread to humans if we could convince Africans to stop eating bush meat. Ditto for HIV btw, I believe most scientists believe it came from someone trying to cook a chimp, not have sex with one. It's amazing that hundreds of millions of painful deaths could probably be traced to a single person handling chimp meat and screwing up.