It's not impossible at all. Mother nature does allow deadly pathogens to propitiate for sure. But what happens is immunity is reached in some, and the variants tend to become more and more contagious but less deadly.

All humans have to do is produce a virus, or unleash a natural one I that has a tendency to mutate fast so that the odds of producing a variant that survives increases.

As long as tons of people are vaccinated against it the chances that it will evolve into a deadly virus by way of evolving in an environment full of vaccine induced antibodies there is a good chance that the virtual s will evolve into a deadly super virus.