[Symmetry]

Our hunter gatherer ancestors living in bands suffered from drastically fewer respiratory diseases than we do, you need a large connected population for something like the flu to survive in a human population in the long term. The issue with sterile environments is about the lack of random bacteria, not human adapted pathogens.

[MontyCarloHall]

That’s likely true, but our hunter gatherer ancestors also saw way more water and foodborne pathogens, not to mention continual parasitic infections to more than make up for the relative lack of immune system stimulation from a lower level of airborne pathogens.