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Dr Marianne Williams, gastroenterology dietician, UK
For roughly the first month of life the immune system is switched off in essence and everything they [babies] get exposed to in that first month in life – dogs, cats, aunts, uncles, grannies, grandpas, family, dirt – everything – that is where they build up all the bacteria that are then going to colonise their gut in the future. Now, if you’re born into a very sterile environment, as is increasingly the case in the western world, everything’s kept terribly clean, and one of the theories is that we just are not getting enough exposure to a variety of bacteria at that very very early stage in that first month of life.