Genetic Mutantations are a fact of Life, why 1918-19 Flu, only killed about 50 million, most had a immunity, those who didn't died ..
In this New World of Super Bugs, it may be our end, Human Race plagued by Rampant Virulent Diseases, until there a small population left, that natural disaster may finish us off ..
Of Course the weak, lightly muscled, enlongated, from weigthlessness, emaciated from calorie restrictive diet (promotes Longevity), may come back from mars or orbit, and pay there respects & lend a hand, but I think Off World Immune Systems will not cope for Sure .. Plus they will look like "greys", we may just shoot them ..
There´s no such thing as superbug, every bug will be vulnerable to something, they may become immune to our current vaccines but we will create new ones. Even bubonic plague only killed 2/3 people it infected and that was the worst epidemic in the known history, enough people will survive that they can repopulate in a short order (a few generations at most) we are not looking at extinction from disease any time soon.