Bacteria are minute, single-celled organisms whose effects on other living things are out of all proportion to their tiny size. Most bacteria are only about one-thousandth of a millimeter in diameter and are a lot smaller other single-celled organisms. But without the help of this vast invisible army, all life would cease to exist. Bacteria can be grouped by the way they live. A few kids make their own food, but most of them live off dead or decaying matter (as saprophytes), or off living organisms (as parasites).