There are many small organisms in the world that we call micro-organisms or microbes. Some are harmful - they may cause illness. Bacteria and viruses may carry diseases. Keeping clean can help prevent these diseases. Keeping our food carefully can help prevent them, too. Micro-organisms live and reproduce on food. Eating poorly-kept or decaying food can cause food poisoning.

But other micro-organisms do a great deal of good. Without them, nothing would decay, and so materials from dead plants and animals would not return to the soil to be used again. Rubbish would pile up forever.

Other micro-organisms help in the production of food. Yeast is a micro-organism - a kind of fungus that produces bubbles of gas that makes bread rise. Bacteria are essential to making yoghurt and cheese.