Bacterial means relating to or caused by bacteria, which are microscopic living things too small to see without a microscope. Bacteria are single-celled organisms found almost everywhere on Earth: in soil, water, air, and even inside your body.

Many people think of bacteria as tiny villains that make you sick, and some do cause bacterial infections like strep throat or ear infections. When doctors prescribe antibiotics, they're giving you medicine that fights these harmful bacteria. A bacterial infection is different from a viral infection (like a cold or flu), which is why antibiotics won't help with viruses.

But most bacteria are either harmless or actually helpful. Your intestines contain trillions of bacteria that help digest food. Other bacteria break down dead plants and animals, returning nutrients to the soil. Still others are used to make foods like yogurt and cheese. Scientists even use bacteria in laboratories to produce medicines like insulin.

When scientists study bacterial cells under a microscope, they're looking at organisms that have lived on Earth for over three billion years, making them some of the oldest and most successful life forms on the planet.