WHEN, at the turn of the century, the first human genomes were sequenced, many biologists felt they had had delivered into their hands the keys to unlocking numerous puzzles about disease. Since then there has indeed been a fruitful effort to understand how the thousands of human genes which control hormones, enzymes and other molecules of the body serve to regulate health. But, in an unexpected turn of events, it is also now apparent that the human genome is not the only one to which attention should be paid. Human guts contain microbes, lots of them. Added together, the genes in these bugs' genomes amount to perhaps 150 times the number in the human genome alone. If the bacteria in question were doing little more than swimming around digesting lettuce, this would be of small consequence. But they are doing much more than that.
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