How did it die in the first place? Is there conclusive evidence that it was the vaccination and not just due to a cumulative effect of better hygiene, better di

How did it die in the first place? Is there conclusive evidence that it was the vaccination and not just due to a cumulative effect of better hygiene, better diet and better living conditions? And when and how did it resurrect itself? These questions need to be answered, because if there was an incorrect assumption in any of the previous answers, the current solution may not be as good as we'd hope...