Bacteria are one kind of microbe. The name bacterium (plural bacteria) comes from the Latin meaning 'rod', as the first ones to be discovered were rod-shaped. Some have wiggling tails.

Microbes are the simplest possible forms of living things. They were the first kinds of life ever to develop on the Earth, some 3-4 billion years ago. For most of the history of life on Earth there has been nothing but microbes. All the large living things you see around you are quite recent compared to microbes such as bacteria.

All microbes are usually so tiny that they cannot be seen. In fact, although they are the world's most common living things, no one even knew they existed until 1675. That is because the word microbe is short for microscopic living things, and until microscopes were invented it was impossible to see them.

There are lots of different kinds of microbes. We know some as bacteria, others as fungi, yet others as viruses. Some grow together in such huge quantities that they turn water green – as is the case with tiny plants called plankton that make water green.

You can't escape the world of microbes because they are in soil, air and water, and they even live deep in rocks of the earth and in the hottest springs and volcanoes.

You mostly don't want to avoid microbes either, because they help your body to work, as well as many other useful things.

Microbes are mostly things that break down dead plants and animals. When things go off, we say they decompose. So microbes are called decomposers. What decomposing does is to return all the substances that the living things are made of so that other living things can use them in their turn. We have bacteria on every surface of our insides. They decompose food. We call it digestion.

So how do they do this? They have no mouths, so they can't chew up dead material. Instead they gradually dissolve it and take in the substances they need through the walls of their bodies, while releasing other substances that we, and other living things, can use.