Later versions of the hygiene hypothesis shifted the emphasis away from pathogens and more towards benevolent microbes that educate our immune systems, or environmental species that lurk in mud and dust, and even parasites that establish long-lasting but tolerable infections. They have been christened ‘old friends’.22 They have been part of our lives throughout our evolutionary history, but their tenure has become shakier of late. Their disappearance isn’t just due to stricter personal cleanliness, as the word ‘hygiene’ unhelpfully implies. It’s also due to the various trappings of
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