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{{More information|Microscopic discovery of bacteria}}
Turkish scientist [[Akshamsaddin]]
{{blockquote|It is incorrect to assume that diseases appear one by one in humans. Disease infects by spreading from one person to another. This infection occurs through seeds that are so small they cannot be seen but are alive.<ref>Taşköprülüzâde: ''Shaqaiq-e Numaniya'', v. 1, p. 48</ref><ref>Osman Şevki Uludağ: ''Beş Buçuk Asırlık Türk Tabâbet Tarihi'' (Five and a Half Centuries of Turkish Medical History). Istanbul, 1969, pp. 35–36</ref>}}
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==Classification and structure==
Microorganisms can be found almost anywhere on [[Earth]]. [[Bacteria]] and [[archaea]] are almost always microscopic, while a number of [[eukaryote]]s are also microscopic, including most [[Protista|protists]], some [[fungus|fungi]], as well as some [[micro-animal]]s and plants. [[Virus]]es are generally regarded as [[Non-cellular life|not living]] and therefore not considered to be microorganisms, although a subfield of [[microbiology]] is [[virology]], the study of viruses.<ref>{{Cite book |title=eLS|last=Lim|first=Daniel V. |date=2001 |publisher=John Wiley |isbn=978-0-470-01590-2 |doi=10.1038/npg.els.0000459|chapter = Microbiology}}</ref><ref>{{Cite web|url=http://www.highveld.com/microbiology/what-is-microbiology.html|title=What is Microbiology? |website=highveld.com |access-date=2017-06-02|archive-url=https://web.archive.org/web/20150215180557/http://www.highveld.com/microbiology/what-is-microbiology.html |archive-date=2015-02-15}}</ref><ref>{{cite book |last=Cann |first=Alan |title=Principles of Molecular Virology |year=2011 |publisher=Academic Press |isbn=978-0-12-384939-7 |edition=5}}</ref>
===Evolution===
{{further|Timeline of
{{PhylomapB||caption=[[Carl Woese]]'s 1990 [[phylogenetic tree]] based on [[rRNA]] data shows the domains of [[Bacteria]], [[Archaea]], and [[Eukaryota]]. All are microorganisms except some eukaryote groups.|size=325px}}
Single-celled microorganisms were the [[Origin of life|first forms of life]] to develop on Earth, approximately 3.5 [[gigaannum|billion years]] ago.<ref>{{Cite journal |author=Schopf, J. |title=Fossil evidence of Archaean life |journal=Philos Trans R Soc Lond B Biol Sci |volume=361 |issue=1470 |pages=869–885 |year=2006 |pmid=16754604 |doi=10.1098/rstb.2006.1834 |pmc=1578735}}</ref><ref>{{Cite journal |author=Altermann, W. |author2=Kazmierczak, J. |title=Archean microfossils: a reappraisal of early life on Earth |journal=Res Microbiol |volume=154 |issue=9 |pages=611–7 |year=2003 |pmid=14596897 | doi=10.1016/j.resmic.2003.08.006|doi-access=free }}</ref><ref>{{Cite journal|author=Cavalier-Smith, T. |author-link=Thomas Cavalier-Smith |title=Cell evolution and Earth history: stasis and revolution |journal=Philos Trans R Soc Lond B Biol Sci |volume=361 |issue=1470 |pages=969–1006 |year=2006 |pmid=16754610 |doi=10.1098/rstb.2006.1842 |pmc=1578732}}</ref> Further evolution was slow,<ref>{{Cite journal| author=Schopf, J. | title=Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic | pmc=44277| journal=PNAS | volume=91 | issue=15 | pages=6735–6742 | year=1994 | pmid=8041691 | doi=10.1073/pnas.91.15.6735 | bibcode=1994PNAS...91.6735S| doi-access=free }}</ref> and for about 3 billion years in the [[Precambrian]] [[Eon (geology)|eon]], (much of the history of [[life|life on Earth]]), all [[organism]]s were microorganisms.<ref>{{Cite journal|author=Stanley, S. |title=An Ecological Theory for the Sudden Origin of Multicellular Life in the Late Precambrian |journal=PNAS |volume=70 |issue=5 |pages=1486–1489 |date=May 1973 |pmid=16592084 |pmc=433525 | doi=10.1073/pnas.70.5.1486 |bibcode=1973PNAS...70.1486S |doi-access=free }}</ref><ref>{{Cite journal |author1=DeLong, E. |author2=Pace, N. | title=Environmental diversity of bacteria and archaea | journal=Syst Biol | volume=50 | issue=4 | pages=470–8 | year=2001 |pmid=12116647 | doi=10.1080/106351501750435040|citeseerx=10.1.1.321.8828 }}</ref> Bacteria, algae and fungi have been identified in [[amber]] that is 220 million years old, which shows that the [[Morphology (biology)|morphology]] of microorganisms has changed little since at least the [[Triassic]] period.<ref>{{Cite journal |author=Schmidt, A. |author2=Ragazzi, E. |author3=Coppellotti, O. |author4=Roghi, G. | title=A microworld in Triassic amber | journal=Nature | volume=444 | issue=7121 | page=835 | year=2006 | pmid=17167469 | doi=10.1038/444835a |bibcode=2006Natur.444..835S |s2cid=4401723 | doi-access=free }}</ref> The newly discovered [[Nickel#Biological role|biological role played by nickel]], however – especially that brought about by [[Types of volcanic eruption|volcanic eruptions]] from the [[Siberian Traps]] – may have accelerated the evolution of [[methanogen]]s towards the end of the [[Permian–Triassic extinction event]].<ref>{{cite web |url= http://www.space.com/26654-microbe-innovation-started-largest-earth-extinction.html |title= Microbe's Innovation May Have Started Largest Extinction Event on Earth |last= Schirber |first=Michael |date= 27 July 2014 |publisher=Astrobiology Magazine |website= Space.com |quote=That spike in nickel allowed methanogens to take off.}}</ref>
Microorganisms tend to have a relatively fast rate of evolution. Most microorganisms can reproduce rapidly, and bacteria are also able to freely exchange genes through [[Bacterial conjugation|conjugation]], [[Transformation (genetics)|transformation]] and [[Transduction (genetics)|transduction]], even between widely divergent species.<ref>{{Cite journal| author=Wolska, K. | title=Horizontal DNA transfer between bacteria in the environment | journal=Acta Microbiol Pol | volume=52 | issue=3 | pages=233–243 | year=2003 |pmid=14743976}}</ref> This [[horizontal gene transfer]], coupled with a high [[mutation]] rate and other means of transformation, allows microorganisms to swiftly [[biological evolution|evolve]] (via [[natural selection]]) to survive in new environments and respond to [[stressors|environmental stresses]]. This rapid evolution is important in medicine, as it has led to the development of [[multidrug resistance|multidrug resistant]] [[pathogenic bacteria]], ''superbugs'', that are [[antimicrobial resistance|resistant to antibiotics]].<ref>{{Cite journal |author=Enright, M. |author2=Robinson, D. |author3=Randle, G. |author4=Feil, E. |author5=Grundmann, H. |author6=Spratt, B. | title=The evolutionary history of methicillin-resistant ''Staphylococcus aureus'' (MRSA) | journal=Proc Natl Acad Sci USA | volume=99 | issue=11 | pages=7687–7692 |date=May 2002 | pmid=12032344 |pmc=124322 | doi=10.1073/pnas.122108599|bibcode=2002PNAS...99.7687E |doi-access=free }}</ref>
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Archaea are [[prokaryote|prokaryotic]] unicellular organisms, and form the first domain of life in [[Carl Woese]]'s [[three-domain system]]. A prokaryote is defined as having no [[cell nucleus]] or other [[lipid bilayer|membrane bound]]-[[organelle]]. Archaea share this defining feature with the bacteria with which they were once grouped. In 1990 the microbiologist Woese proposed the three-domain system that divided living things into bacteria, archaea and eukaryotes,<ref>{{Cite journal |author1=Woese, C. |author1-link=Carl Woese | author2=Kandler, O. | author3=Wheelis, M. | title=Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya | doi= 10.1073/pnas.87.12.4576 | journal=Proc Natl Acad Sci USA | volume=87 | issue=12 | pages=4576–9 | year=1990 | pmid=2112744 | pmc=54159 | bibcode=1990PNAS...87.4576W|doi-access=free }}</ref> and thereby split the prokaryote domain.
Archaea differ from bacteria in both their genetics and biochemistry. For example, while bacterial [[cell membrane]]s are made from [[phospholipid|phosphoglycerides]] with [[ester]] bonds, archaean membranes are made of [[ether lipid]]s.<ref>{{Cite journal |author1=De Rosa, M. |author2=Gambacorta, A. | author3=Gliozzi, A. |title=Structure, biosynthesis, and physicochemical properties of archaebacterial lipids |journal=Microbiol. Rev. |volume=50 |issue=1 |pages=70–80 |date=1 March 1986|pmid=3083222 |pmc=373054 |doi=10.1128/mmbr.50.1.70-80.1986}}</ref> Archaea were originally described as [[extremophile]]s living in [[extreme environment]]s, such as [[hot spring]]s, but have since been found in all types of [[habitat]]s.<ref>{{Cite journal |author1=Robertson, C. |author2=Harris, J. |author3=Spear, J. |author4=Pace, N. | title=Phylogenetic diversity and ecology of environmental Archaea | journal=Curr Opin Microbiol | volume=8 | issue=6 | pages=638–42 | year=2005 | pmid=16236543 | doi=10.1016/j.mib.2005.10.003}}</ref> Only now are scientists beginning to realize how common archaea are in the environment, with [[Thermoproteota]] (formerly Crenarchaeota) being the most common form of life in the ocean, dominating ecosystems below {{convert|150
The combined domains of archaea and bacteria make up the most diverse and abundant group of [[organism]]s on Earth and inhabit practically all environments where the temperature is below +{{convert|140
The biodiversity of the prokaryotes is unknown, but may be very large. A May 2016 estimate, based on laws of scaling from known numbers of species against the size of organism, gives an estimate of perhaps 1 trillion species on the planet, of which most would be microorganisms. Currently, only one-thousandth of one percent of that total have been described.<ref name="NSF-2016002">{{cite news |author=Staff |title=Researchers find that Earth may be home to 1 trillion species |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=138446 |date=2 May 2016 |work=[[National Science Foundation]] |access-date=6 May 2016 }}</ref> [[Archaea|Archael cells]] of some species aggregate and transfer [[DNA]] from one cell to another through direct contact, particularly under stressful environmental conditions that cause [[DNA damage (naturally occurring)|DNA damage]].<ref>{{cite journal |
===Bacteria===
{{Main|Bacteria}}
[[File:Staphylococcus aureus 01.jpg|thumb|''[[Staphylococcus aureus]]'' bacteria magnified about 10,000x]]
Their [[genome]] is usually a [[circular bacterial chromosome]] – a single loop of [[DNA]], although they can also harbor small pieces of DNA called [[plasmid]]s. These plasmids can be transferred between cells through [[bacterial conjugation]]. Bacteria have an enclosing [[Bacterial cell structure#Cell wall|cell wall]], which provides strength and rigidity to their cells. They reproduce by [[binary fission]] or sometimes by [[budding]], but do not undergo [[Meiosis|meiotic]] [[sexual reproduction]]. However, many bacterial species can transfer DNA between individual cells by a [[horizontal gene transfer]] process referred to as natural [[Transformation (genetics)|transformation]].<ref>{{cite journal |author=Johnsbor, O. |author2=Eldholm, V. |author3=Håvarstein, L.S. |title=Natural genetic transformation: prevalence, mechanisms and function |journal=Res. Microbiol. |volume=158 |issue=10 |pages=767–78 |date=December 2007 |pmid=17997281 |doi=10.1016/j.resmic.2007.09.004 |doi-access=free }}</ref> Some species form extraordinarily resilient [[endospore|spores]], but for bacteria this is a mechanism for survival, not reproduction. Under optimal conditions bacteria can grow extremely rapidly and their numbers can double as quickly as every 20 minutes.<ref>{{Cite journal| author=Eagon, R. | title=Pseudomonas Natriegens, a Marine Bacterium With a Generation Time of Less Than 10 Minutes | journal=J Bacteriol | volume=83 | issue=4| pages=736–7 | year =1962 | pmid=13888946 | pmc=279347| doi=10.1128/JB.83.4.736-737.1962 }}</ref>
===Eukaryotes===
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Most living things that are visible to the naked eye in their adult form are [[eukaryote]]s, including [[human]]s. However, many eukaryotes are also microorganisms. Unlike [[bacteria]] and [[archaea]], eukaryotes contain [[organelle]]s such as the [[cell nucleus]], the [[Golgi apparatus]] and [[mitochondrion|mitochondria]] in their [[cell (biology)|cells]]. The nucleus is an organelle that houses the [[DNA]] that makes up a cell's genome. DNA (Deoxyribonucleic acid) itself is arranged in complex [[chromosome]]s.<ref>[http://www.ucmp.berkeley.edu/alllife/eukaryotamm.html Eukaryota: More on Morphology.] (Retrieved 10 October 2006)</ref>
Mitochondria are organelles vital in [[metabolism]] as they are the site of the [[citric acid cycle]] and [[oxidative phosphorylation]]. They evolved from [[symbiotic]] bacteria and retain a remnant genome.<ref name=Dyall>{{Cite journal |author=Dyall, S. |author2=Brown, M. |author3=Johnson, P. | title=Ancient invasions: from endosymbionts to organelles | journal=Science | volume=304 | issue=5668 | pages=253–7 | year=2004|pmid=15073369 | doi=10.1126/science.1094884|bibcode=2004Sci...304..253D |s2cid=19424594
Unicellular eukaryotes consist of a single [[Cell (biology)|cell]] throughout their life cycle. This qualification is significant since most [[multicellular organism|multicellular]] eukaryotes consist of a single cell called a [[zygote]] only at the beginning of their life cycles. Microbial eukaryotes can be either [[haploid]] or [[diploid]], and some organisms have multiple [[cell nucleus|cell nuclei]].<ref>See [[coenocyte]].</ref>
Unicellular eukaryotes usually reproduce asexually by [[mitosis]] under favorable conditions. However, under stressful conditions such as nutrient limitations and other conditions associated with DNA damage, they tend to reproduce sexually by [[meiosis]] and [[Fertilization|syngamy]].<ref name=Bernstein>{{cite book |last1=Bernstein |first1=H. |last2=Bernstein |first2=C. |last3=Michod |first3=R.E. |year=2012 |chapter-url=https://www.novapublishers.com/catalog/product_info.php?products_id=31918 |title=DNA repair as the primary adaptive function of sex in bacteria and eukaryotes. |chapter=Chapter 1 |pages=1–49 |
====Protists====
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[[File:Euglena mutabilis - 400x - 1 (10388739803) (cropped).jpg|thumb|upright|''[[Euglena|Euglena mutabilis]]'', a [[photosynthetic]] [[flagellate]]]]
Of [[Eukaryote|eukaryotic]] groups, the [[protists]] are most commonly [[unicellular]] and microscopic. This is a highly diverse group of organisms that are not easy to classify.<ref>{{Cite journal|author=Cavalier-Smith T |author-link=Thomas Cavalier-Smith |title=Kingdom protozoa and its 18 phyla |journal=Microbiol. Rev. |volume=57 |issue=4 |pages=953–994 |date=1 December 1993|pmid=8302218 |pmc=372943 |doi=10.1128/mmbr.57.4.953-994.1993 |doi-access=free }}</ref><ref>{{Cite journal|author=Corliss JO |title=Should there be a separate code of nomenclature for the protists? |journal=BioSystems |volume=28 |issue=1–3 |pages=1–14 |year=1992 |pmid=1292654 | doi=10.1016/0303-2647(92)90003-H|bibcode=1992BiSys..28....1C }}</ref> Several [[algae]] [[species]] are [[multicellular]] protists, and [[slime mold]]s have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.<ref>{{Cite journal|author=Devreotes P |title=Dictyostelium discoideum: a model system for cell-cell interactions in development |journal=Science |volume=245 |issue=4922 |pages=1054–8 |year=1989 |pmid=2672337 | doi=10.1126/science.2672337|bibcode=1989Sci...245.1054D }}</ref> The number of species of protists is unknown since only a small proportion has been identified. Protist diversity is high in oceans, deep sea-vents, river sediment and an acidic river, suggesting that many eukaryotic microbial communities may yet be discovered.<ref>{{Cite journal|author=Slapeta, J |author2=Moreira, D |author3=López-García, P. |title=The extent of protist diversity: insights from molecular ecology of freshwater eukaryotes |journal=Proc. Biol. Sci. |volume=272 |issue=1576 |pages=2073–2081 |year=2005 |pmid=16191619 |doi=10.1098/rspb.2005.3195 |pmc=1559898}}</ref><ref>{{Cite journal |author=Moreira, D. |author2=López-García, P. |title=The molecular ecology of microbial eukaryotes unveils a hidden world |journal=Trends Microbiol. |volume=10 |issue=1 |pages=31–8 |year=2002 |pmid=11755083 | url=http://download.bioon.com.cn/view/upload/month_0803/20080326_daa08a6fdb5d38e3a0d8VBrocN3WtOdR.attach.pdf | doi=10.1016/S0966-842X(01)02257-0}}</ref>
====Fungi====
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[[file:Deinococcus radiodurans.jpg|thumb|upright|A tetrad of ''[[Deinococcus radiodurans]]'', a [[radioresistant]] [[extremophile]] bacterium]]
[[Extremophiles]] are microorganisms that have adapted so that they can survive and even thrive in [[extreme environment]]s that are normally fatal to most life-forms. [[Thermophile]]s and [[hyperthermophiles]] thrive in high [[temperature]]s. [[Psychrophile]]s thrive in extremely low temperatures. – Temperatures as high as {{convert|130|°C|°F}},<ref>[[Strain 121]], a [[Hyperthermophile|hyperthermophilic]] [[archaea]], has been shown to reproduce at {{convert|121|°C|°F}}, and survive at {{convert|130|°C|°F}}.[https://www.nsf.gov/od/lpa/news/03/pr0384.htm]</ref> as low as {{convert|-17|°C|°F}}<ref>Some [[Psychrophiles|Psychrophilic]] bacteria can grow at {{convert|-17|°C|°F}}),[http://news.bbc.co.uk/1/hi/sci/tech/827063.stm] and can survive near [[absolute zero]]).{{cite web |title=Earth microbes on the Moon |url=https://science.nasa.gov/newhome/headlines/ast01sep98_1.htm |url-status=dead |archive-url=https://web.archive.org/web/20100323224432/http://science.nasa.gov/newhome/headlines/ast01sep98_1.htm |archive-date=23 March 2010 |access-date=2009-07-20}}</ref> [[Halophile]]s such as ''[[Halobacterium salinarum]]'' (an archaean) thrive in high [[Salinity|salt conditions]], up to saturation.<ref>Dyall-Smith, Mike, [http://www.microbiol.unimelb.edu.au/people/dyallsmith/ ''HALOARCHAEA''], University of Melbourne. See also [[Haloarchaea]].</ref> [[Alkaliphile]]s thrive in an [[alkaline]] [[pH]] of about 8.5–11.<ref>{{Cite journal|url=http://jb.asm.org/cgi/reprint/185/2/461.pdf|title=''Bacillus alcalophilus'' can grow at up to pH 11.5|journal=Journal of Bacteriology|date=15 January 2003|volume=185|issue=2|pages=461–465|doi=10.1128/JB.185.2.461-465.2003|last1=Olsson|first1=Karen|last2=Keis|first2=Stefanie|last3=Morgan|first3=Hugh W.|last4=Dimroth|first4=Peter|last5=Cook|first5=Gregory M.|pmid=12511491|pmc=145327}}</ref> [[Acidophile]]s can thrive in a pH of 2.0 or less.<ref>[[Picrophilus]] can grow at pH −0.06.[http://www.rcn.montana.edu/resources/organisms/organisminfo.aspx?nav=11&tid=1298&did=1&nid=82076&lid=9] {{Webarchive|url=https://web.archive.org/web/20100622184325/http://www.rcn.montana.edu/resources/organisms/organisminfo.aspx?nav=11&tid=1298&did=1&nid=82076&lid=9
===Plants and
{{Main |Soil biology}}
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Microorganisms are used in a [[Fermentation (food)|fermentation]] process to make [[yoghurt]], [[cheese]], [[curd]], [[kefir]], [[ayran]], [[fermented milk products|xynogala]], and other types of food. Fermentation cultures provide flavour and aroma, and inhibit undesirable organisms.<ref>{{cite web|url=http://www.foodsci.uoguelph.ca/dairyedu/micro.html |title= Dairy Microbiology |access-date=9 October 2006 |publisher= University of Guelph}}</ref> They are used to [[leavening agent|leaven]] [[bread]], and to convert [[sugar]]s to [[ethanol|alcohol]] in [[wine]] and [[beer]]. Microorganisms are used in [[brewing]], [[wine making]], [[baking]], [[pickling]] and other [[food]]-making processes.<ref name="HuiMeunier-Goddik2004">{{cite book |author=Hui, Y.H. |author2=Meunier-Goddik, L. |author3=Josephsen, J. |author4=Nip, W.K. |author5=Stanfield, P.S. |title=Handbook of Food and Beverage Fermentation Technology |url=https://books.google.com/books?id=PC_O7u1NPZEC&pg=PA27 |year=2004 |publisher=CRC Press |isbn=978-0-8247-5122-7 |pages=27 and passim}}</ref>
{| class="wikitable plainrowheaders"▼
▲{| class="wikitable"
!scope="col"|Product
!scope="col"|Contribution of microorganisms
|-
!scope="row"|Cheese
|Growth of microorganisms contributes to ripening and flavor. The flavor and appearance of a particular cheese is due in large part to the microorganisms associated with it. ''[[Lactobacillus bulgaricus GLB44|Lactobacillus Bulgaricus]]'' is one of the microbes used in production of [[dairy product]]s
|-
!scope="row"|Alcoholic beverages
|yeast is used to convert sugar, grape juice, or malt-treated grain into alcohol. other microorganisms may also be used; a mold converts starch into sugar to make the Japanese rice wine, sake. ''[[Acetobacter aceti|Acetobacter Aceti]]''
|-
!scope="row"|Vinegar
|Certain bacteria are used to convert alcohol into acetic acid, which gives vinegar its acid taste. ''[[Acetobacter aceti|Acetobacter Aceti]]'' is used on production of vinegar, which gives vinegar odor of alcohol and alcoholic taste
|-
!scope="row"|Citric acid
|Certain fungi are used to make citric acid, a common ingredient of soft drinks and other foods.
|-
!scope="row"|Vitamins
|Microorganisms are used to make vitamins, including C, B<sub>2</sub> , B<sub>12.</sub>
|-
!scope="row"|Antibiotics
|With only a few exceptions, microorganisms are used to make antibiotics. ''[[Penicillin]], [[Amoxicillin]], [[Tetracycline]], and [[Erythromycin]]''
|}
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{{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 |
===Disease===
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[[File:Plasmodium.jpg|thumb|upright|The [[eukaryotic]] [[parasite]] ''[[Plasmodium falciparum]]'' (spiky blue shapes), a causative agent of [[malaria]], in human [[blood]]]]
Microorganisms are the causative agents ([[pathogen]]s) in many [[Infection|infectious diseases]]. The organisms involved include [[pathogenic bacteria]], causing diseases such as [[bubonic plague|plague]], [[tuberculosis]] and [[anthrax]]; [[protozoa]]n [[parasite]]s, causing diseases such as [[malaria]], [[African trypanosomiasis|sleeping sickness]], [[dysentery]] and [[toxoplasmosis]]; and also fungi causing diseases such as [[ringworm]], [[candidiasis]] or [[histoplasmosis]]. However, other diseases such as [[influenza]], [[yellow fever]] or [[AIDS]] are caused by [[pathogenic viruses]], which are not usually classified as living organisms and are not, therefore, microorganisms by the strict definition. No clear examples of archaean pathogens are known,<ref>{{Cite journal |author=Eckburg, P. |author2=Lepp, P. |author3=Relman, D. |title=Archaea and Their Potential Role in Human Disease |journal=[[Infect Immun]] |volume=71 |issue=2 |pages=591–6 |year=2003 |pmid=12540534 | doi=10.1128/IAI.71.2.591-596.2003 |pmc=145348}}</ref> although a relationship has been proposed between the presence of some archaean methanogens and human [[periodontal disease]].<ref>{{Cite journal |author=Lepp, P. |author2=Brinig, M. |author3=Ouverney, C. |author4=Palm, K. |author5=Armitage, G. |author6=Relman, D. |title=Methanogenic Archaea and human periodontal disease | doi= 10.1073/pnas.0308766101 | journal=[[Proc Natl Acad Sci USA]] |volume=101 |issue=16 |pages=6176–81 |year=2004 |pmid=15067114 |pmc=395942|bibcode=2004PNAS..101.6176L |doi-access=free }}</ref> Numerous microbial pathogens are capable of sexual processes that appear to facilitate their survival in their infected host.<ref>{{cite journal | author = Bernstein H, Bernstein C, Michod RE | date = Jan 2018 | title = Sex in microbial pathogens | journal = Infect Genet Evol | volume = 57 | pages = 8–25 | doi = 10.1016/j.meegid.2017.10.024 | pmid = 29111273 | doi-access = free | bibcode = 2018InfGE..57....8B }}</ref>
=== Hygiene ===
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* [[Nylon-eating bacteria]]
* [[Petri dish]]
* [[
* [[Budapest Treaty]] (Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure)
{{colend}}
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