Bacteria, also called germs, are microscopic organisms not visible with the naked eye. Bacteria are single-celled, or simple organisms. Though small bacteria are powerful and complex, and they can survive in extreme conditions. Bacteria are when it comes to straight numbers, the biggest population of organisms that exist on Earth.

Bacteria can be found almost anywhere on the planet, deep underground, below the deepest points in the oceans, and even 40 miles high up in the atmosphere. There are many bacteria in the human body also.