ABSTRACT
We live in a microbial world. Ask a friend to name a living thing and he or she will likely pick a dog, a daffodil, or some other familiar animal or plant. The vast majority of living things on this planet, however, are microorganisms-bacteria, viruses, fungi, and protozoa, too small to see with the unaided eye (Figure 1.1a). Just how far these organisms outnumber larger living things is staggering to think about. A single teaspoon of soil can hold millions or billions of bacteria and fungi. A drop of seawater teems with microscopic life. And larger animals and plants provide shelter for literally trillions of smaller living things. At this moment, more bacteria than all the humans that have ever lived are quietly going about their business in your intestine.
