They are far too small to be seen with the naked eye, but bacteria are one of the most widespread forms of lifeand one of the oldest. They appeared on Earth more than 3 billion years before the first dinosaurs. Some bacteria cause deadly diseases, but many are harmless or even helpful to humans and other animals. They fertilize the soil by breaking down dead material into useful chemicals, and they are an essential ingredient in many foods. Bacteria in our own bodies are a vital part of the digestive process. This entertaining book in the You Wouldn't Want to Live Without series shows both the good and the bad sides of bacteria. We couldn't live without them!