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[[File:E coli at 10000x, original.jpg|thumb|upright=1.15|right|A [[Colony (biology)#Microbial colonies|cluster]] of ''[[Escherichia coli]]'' [[bacteria]] magnified 10,000 times]]
{{TopicTOC-Biology}}
A '''microorganism''', or '''microbemoaing''',{{efn|The word ''microorganism'' ({{IPAc-en|ˌ|m|aɪ|k|r|oʊ-|ˈ|ɔːr|g|ə|n|ɪ|z|əm}}) uses [[classical compound|combining forms]] of ''[[wikt:micro-#Prefix|micro-]]'' (from the {{lang-el|μικρός}}, ''mikros'', "small") and ''[[organism]]'' from the {{lang-el|ὀργανισμός}}, ''organismós'', "organism"). It is usually written as a single word but is sometimes [[hyphen]]ated (''micro-organism''), especially in older texts. The informal synonym ''microbe'' ({{IPAc-en|ˈ|m|aɪ|k|r|oʊ|b}}) comes from μικρός, mikrós, "small" and βίος, bíos, "[[life]]".}} is an [[organism]] of [[microscopic scale|microscopic]] size, which may exist in its [[unicellular organism|single-celled]] form or as a [[Colony (biology)#Microbial colonies|colony of cells]].
The possible existence of unseen microbial life was suspected from ancient times, such as in [[Jain scriptures]] from sixth century BC India. The scientific study of microorganisms began with their observation under the [[microscope]] in the 1670s by [[Anton van Leeuwenhoek]]. In the 1850s, [[Louis Pasteur]] found that microorganisms caused [[food spoilage]], debunking the theory of [[spontaneous generation]]. In the 1880s, [[Robert Koch]] discovered that microorganisms caused the diseases [[tuberculosis]], [[cholera]], [[diphtheria]], and [[anthrax]].
==Human health==
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=== Human gut flora ===
{{Further|Human microbiota |Human Microbiome Project}}
Microorganisms can form an [[Endosymbiont|endosymbiotic]] relationship with other, larger organisms. For example, [[Microbial symbiosis and immunity|microbial symbiosis]] plays a crucial role in the immune system. The microorganisms that make up the [[gut flora]] in the [[gastrointestinal tract]] contribute to gut immunity, synthesize [[vitamin]]s such as [[folic acid]] and [[biotin]], and ferment complex indigestible [[carbohydrate]]s.<ref>{{Cite journal |author=O'Hara, A. |author2=Shanahan, F. | title=The gut flora as a forgotten organ | journal=EMBO Rep | volume=7 | issue=7 | pages=688–93 | year=2006 | pmid=16819463 | doi=10.1038/sj.embor.7400731 | pmc=1500832}}</ref> Some microorganisms that are seen to be beneficial to health are termed [[probiotic]]s and are available as [[dietary supplement]]s, or [[food additive#Fortifying agents|food additives]].<ref name=who>{{cite web|last1=Schlundt |first1=Jorgen |title=Health and Nutritional Properties of Probiotics in Food including Powder Milk with Live Lactic Acid Bacteria |url=https://www.who.int/foodsafety/publications/fs_management/en/probiotics.pdf |work=Report of a Joint FAO/WHO Expert Consultation on Evaluation of Health and Nutritional Properties of Probiotics in Food Including Powder Milk with Live Lactic Acid Bacteria |publisher=FAO / WHO |access-date=17 December 2012 |url-status=dead |archive-url=https://web.archive.org/web/20121022161702/http://www.who.int/foodsafety/publications/fs_management/en/probiotics.pdf |archive-date=22 October 2012 }}</ref>
===Disease===
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