To find out, Tom McDade of Northwestern University in Evanston, Illinois, and colleagues turned to health surveys, which began at birth, of 1534 children in Cebu City in the Philippines, where western levels of sanitation are generally not found. When these people reached 20, McDade's team were able to test their blood for C-reactive protein (CRP), a marker of chronic inflammation.
They found that the more pathogens the people had encountered before age 2, the less CRP they had at age 20. Every episode of diarrhoea back then cut the chance of higher CRP later by 11 per cent; every two months spent in a place with animal faeces cut it by 13 per cent. Being born in the dusty, dirty dry season cut the chance by a third (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2009.1795).
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