Family-size clue
Strachan's 1989 paper used birth-order and sibship as a proxy for early infections.
Identity is a correction to advertising: sterile is not the same as healthy, and filthy is not the same as wise.
The hypothesis has been revised toward microbial diversity and regulatory training, sometimes called Old Friends.
Traits
Old Friends
Rook shifted the focus from childhood infections to organisms that were ordinary for millennia.
Clean-water win
Filtration and chlorination crushed waterborne killers. That is not the exposure anyone should restore.
Diversity, not dirt
A narrow indoor microbiome plus antibiotics can be a different story from playing in safe green space.
Regulation
The hoped-for mechanism is better-trained regulatory T cells, not a tougher person in a moral sense.
Not a dare
Seeking infection on purpose is not the lesson. Neither is skipping vaccines.
Clean water stays on the pedestal. The question is which exposures we lost that once calibrated immunity.
Keep exploring
More in Change & defence
Mutation and variants
Populations of genomes, not cartoon villains — RNA viruses copy sloppily, and public-health names track clusters that spread.
Familiarity 92Antibiotic resistance
A shared drug budget under selection in hospitals, farms and wastewater — stewardship, not a lab manual.
Familiarity 85Vaccines and immunity
Antibodies, memory cells and community arithmetic — conceptual, not a prescription and not an anti-vaccine tract.
Familiarity 94Biofilms
Surface cities of microbes — plaque and catheters explain persistence, without a grow-your-own kit.
Familiarity 38Horizontal gene transfer
Sideways DNA traffic — conjugation, transduction, transformation as names and history, not protocols.
Familiarity 45