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It's not just dirt. Traditionally people lived in close proximity to a variety of domestic animals. Most viruses originated in animals and then crossed to humans. So our immune systems evolved to cope with that exposure.

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I personally doubt we need zoonotic diseases to calibrate a healthy immune response. I also believe the close association of people and animals was more salient to European populations than elsewhere, and furthermore we didn’t evolve in an environment where you could expose or be exposed to thousands of people on a regular basis. So I’m not sure I’m convinced by an anthropological argument here.

And IIRC, the hygiene hypothesis mostly has to do with autoimmune diseases and our immune systems not learning how to differentiate between serious threats and harmless exposure.

The word vaccine literally means “of the cow”

That’s because the first vaccine was developed from cow ulcers, not because we need to be near cows.

Mass domestication of animals is also pretty recent on the scale of human existence, on the order of 10-11,000 years ago.

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