“We’re all sterile until we’re born,” says Glenn Gibson, a microbiologist at the University of Reading in Britain. “We haven’t got anything in us right up until the time we come into this big, bad, dirty world.”

But as soon as we pass out of the birth canal, when we are fetched by a doctor’s hands, placed in a hospital crib, put on our mother’s breast, when we drag a thumb across a blanket and stick that thumb in our mouths, when we swallow our first soft food, we are invaded by all sorts of bacteria. Once inside, they multiply – until the bacteria inside us outnumber our human cells.
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University of Chicago immunologist Alexander Chervonsky, with collaborators from Yale University, recently reported that doses of the right stomach bacteria can stop the development of type 1 diabetes in lab mice. “By changing who is living in our guts, we can prevent type 1 diabetes,” he told The Wall Street Journal.

The bottom line: We now have two sets of genes to think about – the ones we got from our parents and the ones of organisms living inside us. Our parents’ genes we can’t change, but the other set? Now that is one of the newest and most exciting fields in cell biology.

You could have two people sitting down to a bowl of cheerios, they could each eat the same number of cheerios but because of a difference in their gut bacteria one will get more calories than the other.