Bacteria are microscopic living things made of just a single cell. They're so tiny you need a microscope to see them, yet they're everywhere: in soil, water, air, and even inside your body right now.

Don't panic! Most bacteria are either harmless or actually helpful. Your digestive system contains trillions of bacteria that help break down food. Bacteria in soil help plants grow by breaking down dead leaves and releasing nutrients. Some bacteria even help make foods like yogurt, cheese, and pickles.

But some bacteria do cause problems. When you get strep throat or an ear infection, harmful bacteria are usually the culprits. These disease-causing bacteria are sometimes called germs. Before doctors understood bacteria, even small cuts could become dangerous infections. The discovery of antibiotics, medicines that kill bacteria, was one of medicine's greatest breakthroughs.

Bacteria have existed for over three billion years, making them among Earth's oldest life forms. Scientists who study bacteria are called bacteriologists. They've discovered that bacteria can survive almost anywhere: in Antarctic ice, in boiling hot springs, even miles underground. Despite being single cells, bacteria are remarkably tough and adaptable, which helps explain why they've thrived on Earth far longer than humans have existed.