Over time, the babies' immune systems start to distinguish between friend and foe in the microbial world, and launch attacks on potentially harmful bugs while leaving beneficial ones, like those that live in the gut, alone.

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She is also studying a group of children to see how early M. smithii buildup occurs, and how soon in development it starts to set up a pattern of weight gain that might then be interrupted by changing the composition of the gut microbial world.

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