@Mr. Tea thinks the human race came into being with the Enlightenment.
We're quite resilient to pathogens thanks to millenia of evolutionary selection.
In the same way that we're selecting for nastier bacteria by current sanitisation practice.
Well, NY could have it but not the rest of the country, that kind of thing, doesn't have to be the whole world.
I think the idea is that a large enough subset is immune to slow or prevent viral transmission among the still susceptible.
My workplace has herd immunity afaict: plenty of cases...
Even better: set up special shopping hours for key workers and oldies that matchmake the most likely to be infected (and furthermore active while infected as they have important roles) with the most susceptible.