Microorganisms (also called microbes) are living things that are too small to see without a microscope. Two important groups are bacteria and some fungi, such as yeast and mould. A patch of mould on old bread is a mass of fungus that has grown big enough for us to see.

Helpful microorganisms: yeast makes bread rise by giving off bubbles of carbon dioxide gas. Some bacteria turn milk into yogurt and cheese. Decomposers (bacteria and fungi) break down dead plants and animals and return nutrients to the soil. Bacteria living in our gut help us digest food.

Harmful microorganisms: some bacteria and fungi spoil food, making it unsafe to eat. Some microbes cause illnesses. Germs is an everyday word for microbes that can make us sick (viruses, which cause colds and flu, are germs too).

Most microorganisms grow fastest when they have food, moisture (water) and warmth. That is why we keep food in the fridge, cover it, and cook it properly. Washing our hands with soap and water, and covering coughs and sneezes, helps stop germs spreading.