A type of prokaryotic microorganism. Unlike eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Bacteria were among the first life forms to evolve on Earth, and can be found in most every habitat, including soil, water, acidic hot springs, radioactive waste, the deep biosphere of the earth’s crust, and in and on other living organisms as symbionts and parasites. Bacteria can be beneficial, such as those comprising the gut flora, or pathogenic and cause infectious disease. However, the vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system.