A microorganism is a living thing so tiny that you need a microscope to see it. Scientists often call them microbes for short.

Microorganisms include bacteria and single-celled organisms like amoebas and paramecia. They live everywhere: in soil, water, air, and even inside your body. Right now, billions of helpful bacteria in your stomach are breaking down your food and helping keep you healthy. Other microorganisms help decompose fallen leaves, turning them into rich soil that feeds new plants.

Not all microorganisms are helpful. Some bacteria cause illness, which is why washing your hands matters: soap and water wash away harmful microbes before they can make you sick. But most microorganisms either help us or simply go about their business without affecting us at all.

Scientists didn't know microorganisms existed until the 1670s, when Dutch scientist Antonie van Leeuwenhoek built one of the first microscopes and discovered a hidden world of tiny living things. His discovery changed medicine and biology, helping scientists understand diseases and develop antibiotics and vaccines. Today, studying microorganisms helps us make cheese and yogurt, clean up oil spills, and even understand more about how life on Earth began.