Many living things are so small that they can only be seen through a microscope. These living things are called microorganisms or microbes. There are three main types of microbe:

• fungi

• bacteria

• viruses

Fungi

Mushrooms and toadstools are fungi, but these are made of lots of cells, so they are not microbes. Yeasts are single-celled fungi, so they are microbes. Fungi are usually the biggest type of microbe. If there is just one of them, we call it a fungus.

Bacteria

Bacteria are usually smaller than fungi. If there is just one of them, we call it a bacterium. Bacteria have many different shapes. Some have 'tails' (called flagella) that let them swim.

Viruses

Viruses are the smallest type of microbe. As a virus can only reproduce inside a cell, some people are not convinced that viruses are really living things.

Differences between fungi, bacteria and viruses

The table shows some of the similarities and differences between the three types of microbe.

Feature	Fungi	Bacteria	Viruses
Cell membrane
Cell wall	(hard)	(soft)	(protein coat)
Cell nucleus		(circle)	(strand)