>The tumor and the dogs aren’t competing against each other anymore,” Baez-Ortega says. “They’re coexisting. The cells behave like cancer cells, but the ecology of the tumor is that of a parasite.” And a fairly benign one at that."

This is pretty much the story of any “old” virus; new viruses (new mutations, species jumping, etc) tend to kill, but they optimize towards coexistence, since killing the host reduces reproduction/spread opportunity.

So eventually they become benign within a population (both by their evolution, and adaptation of human immune system) and only become threatening again when encountering a new population (eg an “infected” city-man visiting rural populations).

Notably, due to the more rapid spread of disease in cities, due to higher density, city-folk tend to have more benign/evolved diseases than rural populations.

Which I think is also the answer to my childhood question: why did the europeans famously have diseases to spread and kill native americans with.. but the native americans could offer no such devastating disease in return? Both groups should have been encountering new diseases; my answer being that the native americans were carrying fewer suppressed diseases.

There’s of course the popular answer that europeans were more evil/willing to spread disease with things like chicken-pox blankets.. (and catapulting rotting animals), but it’s really difficult to imagine native americans would be so primitive that couldn’t learn/mimick the idea... or perhaps its just written out of the history books..

But I never found confirmation on this