Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Bacteria are microscopic single-celled prokaryotic organisms that reproduce by binary fission. Measuring only a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Despite their minute size, they are the most abundant organisms in the world, growing in nearly every environment - in soil, water, and deep in the Earth's crust, even in acidic hot springs and radioactive waste as well as in organic matter and the live bodies of plants and animals. Although the vast majority of bacteria are harmless or beneficial, quite a few bacteria are pathogenic. Learn more about bacteria and the many various diseases they can cause in the following pages.