A bacterium is a tiny living thing made of just one cell, so small you need a microscope to see it. The word comes from ancient Greek, meaning “little stick,” because early scientists looking through microscopes noticed that many bacteria had stick-like shapes.

Bacteria are absolutely everywhere: in soil, in water, in the air, on your skin, and inside your body. Most bacteria are harmless or even helpful. The bacteria in your stomach help digest food. Bacteria in soil break down dead leaves and return nutrients to the earth. Without bacteria, life on Earth would be impossible.

Some bacteria do cause disease. When you get strep throat or an ear infection, harmful bacteria have multiplied in your body. That's when doctors prescribe antibiotics, medicines specifically designed to kill bacteria without harming your own cells.

Scientists have discovered thousands of types of bacteria, each with different shapes, abilities, and roles. Some bacteria can survive in boiling water. Others live in frozen Antarctic ice. A few can even eat oil or produce light.

The plural of bacterium is bacteria. You might hear someone say “bacteria are everywhere” (plural) or “a bacterium was found in the sample” (singular). People sometimes use bacteria for both, but now you know the correct forms.