Microbes are the oldest living organisms on the planet. They have been around for billions of years. Why haven't you seen them, then? Microbes are tiny, single-celled organisms--millions could fit in the eye of a needle. You might think of microbes as organisms that make us sick. This is true, but it is also true that we couldn't live without them. Readers will be thrilled to learn more about the microscopic world of microbes and their impact on the human body.