Bacteria are tiny living organisms that cannot be seen without a microscope. They are found almost everywhere on Earth. This includes soil, water, air, and inside living bodies.

Some bacteria help with digestion in the human body, while others are used in making foods like yogurt or cheese. Other types can cause illnesses such as infections. Bacteria can reproduce very quickly, sometimes doubling in number within minutes under the right conditions.

Bacteria are studied in microbiology. Scientists have identified millions of different types living in many environments.