Bacteria can be found almost everywhere on Earth, including in soil, water, air, and within the bodies of plants and animals. Within the body, bacteria may be found on the skin, in the airways and mouth as well as in the digestive system, urinary tract, and reproductive system. Friendly bacteria make up the microbiome, which play essential roles in processes such as decomposition, nutrient cycling, and digestion. 

Some species of bacteria are pathogens or parasites that cause diseases in animals and plants.