Microbes fall into five big groups. Four of them are made of cells, and one is not.

• Bacteria: single cells, usually 0.5 to 5 µm, with no nucleus. DNA floats in the cell. They live almost everywhere: soil, water, your gut and your skin.
• Archaea: single cells that look like bacteria under a microscope but are a separate branch of life. Many live in extreme places such as hot springs and very salty lakes, but they are also found in soil, oceans and the human gut. No archaeon has been clearly shown to cause an infectious disease in humans.
• Fungi: cells with a nucleus. Yeasts are single cells, moulds grow as threads. Yeast makes bread rise.
• Protists: a mixed group of mostly single-celled organisms with a nucleus, such as amoebae and Paramecium.
• Viruses: not cells at all. A virus is a package of genes in a protein coat. It cannot make copies of itself without borrowing a living cell.