Chances are, most adults see the word "microbes" and link it to one disease or other. This introduction to the infinitesimal creatures that dominate our world (twenty million to "a single drop of seawater"), by zoologist and author Nicola Davies (Surprising Sharks, Extreme Animals) and illustrator Emily Sutton, is an engaging eye-opener. Microbes, Davies informs us, "live everywhere," in volcanoes and rocks, "at the back of your fridge…" and "on the outside and inside" of all living things. Some make us sick, some keep us well, some help us breathe, some "wear down mountains and build up cliffs." They are also "invisible transformers," Davies notes, as food becomes compost, milk turns into yogurt, rocks become soil. And a few can make us sick indeed. In words and pictures, this is fascinating stuff, presented with child-appropriate clarity and a sense of shared discovery.