Human Cells Make Up Only Half Our Bodies. A New Book Explains Why.

I CONTAIN MULTITUDES The Microbes Within Us and a Grander View of Life By Ed YongIllustrated. 355 pp. Ecco/HarperCollins Publishers. $27.99. Reader, as you read these words, trillions of microbes and quadrillions of viruses are multiplying on your face, your hands and down there in the darkness of your gut.