Two terms, side by side
Hygiene hypothesis vs old friends hypothesis
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Hygiene hypothesis
David Strachan's 1989 suggestion that childhood infection carried by older siblings protects against hay fever. The label misled: what protects appears to be animals, siblings, farm dust and outdoor microbes.
Old friends hypothesis
Graham Rook's replacement for the hygiene hypothesis: immune regulation depends on organisms present throughout mammalian evolution, such as environmental mycobacteria, worms and lactobacilli, now largely gone from cities.
Both terms come from Topic 18, The Immune System's Education, which carries the evidence behind them. The gut and immunity, as a topic.