Hygiene and balance in : Clean-water win · 465

Clean water saves lives; missing old microbial partners is a separate, still-debated immune story. David Strachan noted less hay fever in larger families and coined the hygiene hypothesis in the BMJ. Graham Rook argued we need microbes we co-evolved with — not street dirt or a skipped vaccine.

Chlorination and sewage cut cholera and typhoid. That success is not cancelled by allergy trends. European farm-child studies linked livestock exposure and raw-milk contexts to lower atopy — correlation with caveats. As worms and some infections fell in rich countries, some immune-mediated diseases rose — a pattern, not a prescription to seek worms.

The useful claim is missing old partners and diversity, not that open sewers or unwashed hands are medicine. The hygiene hypothesis began as an observation about family size and hay fever, not as a slogan that dirt is medicine. Clean water, sewers and vaccines saved more lives than any romantic idea of a muddy childhood. Balance means keeping those wins while asking why some immune diseases rose.