Similarly Smallpox and chickenpox are probably the same disease.

The working understanding of viruses is basically the same as bacteria, except without the photographic or video evidence. In which case, unless exposed to a population that has no natural immunity, or a major mutation by utter chance such as a cross species jump, the general trend of diseases should be to evolve to being less deadly. As well as to evolve to avoid mechanisms that prevent their ability to reproduce.

So smallpox just evolving to be less deadly over the years makes sense, while a mechanism like a vaccine making it extinct doesn't. Especially since such a mechanism is fairly static.