Aquatic microbiology is the science that deals with microscopic living organisms in fresh or salt water systems. While aquatic microbiology can encompass all microorganisms, including microscopic plants and animals, it more commonly refers to the study of bacteria, viruses, and fungi and their relation to other organisms in the aquatic environment. Bacteria, viruses, and fungi are widely distributed throughout aquatic environments. They can be found in fresh water rivers, lakes, and streams, in the surface waters and sediments of the world's oceans, and even in hot springs. They have even been found supporting diverse communities at hydrothermal vents in the depths of the oceans. Humans have taken advantage of the role these microorganisms play in nutrient cycles. At sewage treatment plants, microscopic bacteria are cultured and then used to break down human wastes. However, in addition to the beneficial uses of some aquatic microorganisms, others may cause problems for people because they are pathogens, which can cause serious diseases. For example, viruses such as Salmonella typhi, S. paratyphoid, and the Norwalk virus are found in water contaminated by sewage can cause illness. Fecal coliform (E. coli) bacteria and Enterococcus bacteria are two types of microorganisms that are used to indicate the presence of disease causing microorganisms in aquatic environments.