imagine if you will, space travelers from earth landing on an alien planet and being greeted and welcomed by the alien inhabitants, now jump forward 50 years and 3/4 of the population is dead or dying from smallpox,measles, and all the the cooties that are living in our bodies, bunkum , pish posh, ridiculous duckin, maybe. humor me a bit but lets jump in professor duckins way back machine, back 100 or more years to early explorers here, south america, africa, greeting villages of happy families, and years later entire villages wiped out from deseases we brought in. ok now lets jump back into our machine and shoot forward to 2020, somewhere in wuhan china a virus thats totally alien to humans has been created, somehow this virus escapes from pandoras box and is running loose in china killing thousands, eventually it finds its way to other countries killing folks. But wait its not killing everyone or killing everyone in the same way, because its alien its mutating, its changing to suit its host and mankind is vexed because he cant pin the little bugger down like the seasonal flu, and he never will. Herd immunity will not work here because the virus is always mutating and will continue to do so because its manmade not natural

Cheech and Chong... and Duck.

I need a bag of that good stuff.

When the Europeans arrived most of the population had already been wiped out. The small-pox came after there had already been a massive reduction in population due to disease. There are Aztec or Mayan, I forget, records stating this.

The only mutations that matter are those where the markers on the virus's coat are no longer similar sufficiently for the antibodies to bind to them and mark them. (This is why cowpox was a good enough analog for smallpox).