Editing Pathogen transmission

{{Short description|Passing of a pathogen from one organism to another}}
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{{About|transmission of disease-causing pathogens|other uses| Transmission (disambiguation){{!}} Transmission |}}
{{Use dmy dates|date=May 2020}}
In [[medicine]], [[public health]], and [[biology]], '''transmission''' is the passing of a [[pathogen]] causing [[Infectious disease|communicable disease]] from an infected [[host (biology)|host]] individual or group to a particular individual or group, regardless of whether the other individual was previously infected.<ref>{{cite book | vauthors = Bush AO, Fernández JC, Esch GW, Seed JR |title=Parasitism: The Diversity and Ecology of Animal Parasites |date=2001 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521664479 |pages=391–399}}</ref> The term strictly refers to the transmission of [[microorganism]]s directly from one individual to another by one or more of the following means:<!--cited in body of article-->
* [[airborne transmission]] – very small dry and wet particles that stay in the air for long periods of time allowing airborne contamination even after the departure of the host.  Particle size < 5 [[μm]].
* [[Respiratory droplet|droplet transmission]] – small and usually wet particles that stay in the air for a short period of time. Contamination usually occurs in the presence of the host. Particle size > 5 μm.
* direct physical contact – touching an infected individual, including sexual contact
* indirect physical contact – usually by touching a contaminated surface, including [[soil]] ([[fomite]])
* [[fecal–oral route|fecal–oral transmission]] – usually from unwashed hands, contaminated food or water sources due to lack of [[sanitation]] and [[hygiene]], an important transmission route in pediatrics, veterinary medicine and developing countries.
* via contaminated hypodermic needles or blood products

Transmission can also be indirect, via another [[organism]], either a [[Vector (epidemiology)|vector]] (e.g. a [[mosquito]] or fly) or an intermediate host (e.g. [[Taenia solium|tapeworm in pigs]] can be transmitted to humans who ingest improperly cooked [[pork]]). Indirect transmission could involve [[zoonosis|zoonoses]] or, more typically, larger pathogens like [[macroparasite]]s with more complex [[Biological life cycle|life cycle]]s. Transmissions can be [[autochthonous transmission|autochthonous]] (i.e. between two individuals in the same place) or may involve travel of the microorganism or the affected hosts.

A 2024 World Health Organization report standardized the terminology for the transmission modes of all respiratory pathogens in alignment with particle physics: airborne transmission; inhalation; direct deposition; and contact.<ref name=":4">{{Cite journal |last=Leung |first=Nancy H. L. |last2=Milton |first2=Donald K. |date=2024-07-03 |title=New WHO proposed terminology for respiratory pathogen transmission |url=https://www.nature.com/articles/s41579-024-01067-5 |journal=Nature Reviews Microbiology |language=en |pages=1–2 |doi=10.1038/s41579-024-01067-5 |issn=1740-1534|url-access=subscription }}</ref> But these newly standardized terms have yet to be translated to policy, including infection control policy<ref name=":4" /> or the pandemic accords or updated International Health Regulations.

==Definition and related terms==
An [[infection|infectious disease]] agent can be transmitted in two ways: as {{visible anchor|horizontal}} disease agent transmission from one individual to another in the same generation (peers in the same age group)<ref>{{cite web | url = http://www.online-medical-dictionary.org/Horizontal+Disease+Transmission.asp?q=Horizontal+Disease+Transmission | title = Horizontal Disease Transmission | archive-url = https://web.archive.org/web/20070927140147/http://www.online-medical-dictionary.org/Horizontal+Disease+Transmission.asp?q=Horizontal+Disease+Transmission | archive-date=27 September 2007 | work = online-medical-dictionary.org }}</ref>  by either direct contact (licking, touching, biting), or indirect contact through air – cough or sneeze ([[Vector (epidemiology)|vectors]] or [[fomites]] that allow the transmission of the agent causing the disease without physical contact)<ref>{{cite web | url = http://www.vet.uga.edu/vpp/archives/IVM/ENG/Modes/routes.htm | title = Routes of transmission of infectious diseases agents | archive-url = https://web.archive.org/web/20120315032413/http://www.vet.uga.edu/vpp/archives/IVM/ENG/Modes/routes.htm | archive-date=15 March 2012 | work = Modes of Introduction of Exotic Animal Disease Agents | vauthors = Kurkjian KM, Little SE | publisher = The University of Georgia College of Veterinary Medicine }}</ref>
or by [[vertical disease transmission]], passing the agent causing the disease from parent to offspring, such as in prenatal or [[perinatal transmission]].<ref>{{cite web | url = http://www.medterms.com/script/main/art.asp?articlekey=7982 | title = Vertical transmission | work = medterms.com | archive-url = https://web.archive.org/web/20070328061254/http://www.medterms.com/script/main/art.asp?articlekey=7982 | archive-date=28 March 2007 }}</ref>

The term '''[[infectivity]]''' describes the ability of an [[organism]] to enter, survive and multiply in the host, while the ''infectiousness'' of a disease agent indicates the comparative ease with which the disease agent is transmitted to other hosts.<ref>{{cite web | url=http://www.doh.wa.gov/notify/other/glossary.htm | title=Glossary of Notifiable Conditions | publisher=Washington State Department of Health | access-date=3 February 2010 | url-status=dead | archive-url=https://web.archive.org/web/20100107002810/http://www.doh.wa.gov/notify/other/glossary.htm | archive-date=7 January 2010 }}</ref> Transmission of pathogens can occur by direct contact, through contaminated food, body fluids or objects, by airborne inhalation or through [[Vector (epidemiology)|vector]] organisms.<ref name=Sherris>{{cite book | veditors = Ryan KJ, Ray CG | title = Sherris Medical Microbiology | edition = 4th | publisher = McGraw Hill | year = 2004 | isbn = 978-0-8385-8529-0 }}</ref>

'''Transmissibility''' is the [[probability]] of an infection, given a contact between an infected host and a noninfected host.<ref>{{cite web | vauthors = Jones JH | title = Notes on R<sub>0</sub> | url = https://web.stanford.edu/~jhj1/teachingdocs/Jones-on-R0.pdf | location = Palo Alto, California | publisher = Department of Anthropological Sciences, Stanford University | date = May 2007 | volume = 323 | pages = 1–9 }}</ref>

'''Community transmission''' means that the source of infection for the spread of an illness is unknown or a link in terms of contacts between patients and other people is missing. It refers to the difficulty in grasping the epidemiological link in the community beyond confirmed cases.<ref>{{cite web |title=Novel Coronavirus (COVID-19) Resources |url=https://asm.org/Press-Releases/2020/COVID-19-Resources}}</ref><ref>{{cite web |title=Gainers and losers in the time of nCoV |url=https://www.manilatimes.net/2020/02/10/opinion/columnists/gainers-and-losers-in-the-time-of-ncov/681991/ |website=The Manila Times |date=10 February 2020}}</ref><ref>{{cite web |title=Global economy looks woozy |url=https://www.pressreader.com/usa/the-dallas-morning-news/20200228/281479278453790 |work=[[Dallas Morning News]] |date=28 February 2020 |via=pressreader.com}}{{subscription required}}</ref>

'''Local transmission''' means that the source of the infection has been identified within the reporting location (such as within a country, region or city).<ref name="whositrep47">{{Cite web|url= https://www.who.int/docs/default-source/coronaviruse/situation-reports/20200307-sitrep-47-covid-19.pdf?sfvrsn=27c364a4_4|title=Coronavirus disease 2019 (COVID-19) Situation Report – 47|website=World Health Organization |access-date=8 March 2020|archive-url= https://web.archive.org/web/20200308130605/https://www.who.int/docs/default-source/coronaviruse/situation-reports/20200307-sitrep-47-covid-19.pdf?sfvrsn=27c364a4_4 |archive-date=8 March 2020 |url-status=live}}</ref>

==Routes of transmission==

The route of transmission is important to [[epidemiology|epidemiologists]] because patterns of contact vary between different populations and different groups of populations depending on socio-economic, cultural and other features. For example, low personal and food hygiene due to the lack of a clean water supply may result in increased transmission of diseases by the fecal-oral route, such as [[cholera]]. Differences in incidence of such diseases between different groups can also throw light on the routes of transmission of the disease. For example, if it is noted that [[polio]] is more common in cities in underdeveloped countries, without a clean water supply, than in cities with a good plumbing system, we might advance the theory that polio is spread by the fecal-oral route. Two routes are considered to be [[Airborne disease|airborne]]: Airborne infections and droplet infections.{{cn|date=June 2021}}

===Airborne infection===
{{Main|Airborne disease}}
"Airborne transmission refers to infectious agents that are spread via droplet nuclei (residue from evaporated droplets) containing infective microorganisms. These organisms can survive outside the body and remain suspended in the air for long periods of time. They infect others via the upper and lower respiratory tracts."<ref name=nhmrc>{{cite web |author=NHMRC|author-link=National Health and Medical Research Council|date=2010|publisher=Commonwealth of Australia|url=https://www.nhmrc.gov.au/sites/default/files/documents/attachments/publications/icg-clinical-ed-guide.pdf |title=Clinical Educators Guide for the Prevention and Control of Infection in Healthcare |access-date=10 November 2020}}</ref> The size of the particles for airborne infections need to be < 5 μm.<ref name=":0">{{Cite web|url=http://apps.who.int/medicinedocs/documents/s16355e/s16355e.pdf|title=Prevention of hospital-acquired infections|work=World Health Organization (WHO)|url-status=dead|archive-url=https://web.archive.org/web/20200326160913/https://apps.who.int/medicinedocs/documents/s16355e/s16355e.pdf |archive-date=26 March 2020 }}</ref> It includes both dry and wet aerosols and thus requires usually higher levels of isolation since it can stay suspended in the air for longer periods of time. i.e., separate ventilation systems or negative pressure environments are needed to avoid general contamination. e.g., [[tuberculosis]], [[chickenpox]], [[measles]].{{cn|date=June 2021}}

==={{visible anchor|Droplet}} infection===
{{Main|Respiratory droplet}}
[[File:Sneeze.JPG|alt=Droplet image captured under dark background on scattering illumination or tyndall effect|thumb|Respiratory droplets are released through talking, coughing, or sneezing.<ref name="canada">{{Cite web |url=http://www.ccohs.ca/oshanswers/prevention/respiratory_protection.html |title=Respiratory Protection Against Airborne Infectious Agents for Health Care Workers: Do surgical masks protect workers? |date=28 February 2017 |publisher=Canadian Centre for Occupational Health and Safety |type=OSH Answers Fact Sheets  |access-date=28 February 2017}}</ref>]]

A common form of transmission is by way of respiratory droplets, generated by [[coughing]], [[sneezing]], or talking. Respiratory droplet transmission is the usual route for respiratory infections. Transmission can occur when respiratory droplets reach susceptible mucosal surfaces, such as in the eyes, nose or mouth. This can also happen indirectly via contact with [[Fomite|contaminated surfaces]] when hands then touch the face. Before drying, respiratory droplets are large and cannot remain suspended in the air for long, and are usually dispersed over short distances.<ref name=nhmrc/> The size of the particles for droplet infections are > 5 μm.<ref name=":0" />

Organisms spread by droplet transmission include respiratory viruses such as [[influenza virus]], [[parainfluenza virus]], [[adenoviruses]], [[rhinovirus]], [[respiratory syncytial virus]], [[human metapneumovirus]], ''[[Bordetella pertussis]]'', [[pneumococci]], [[streptococcus pyogenes]], [[diphtheria]], [[rubella]],<ref>{{cite web|url=http://www.rightdiagnosis.com/d/diseases_contagious_from_droplets/basics.htm|title=What is Diseases contagious from droplets?|url-status=live|archive-url=https://web.archive.org/web/20150716185815/http://www.rightdiagnosis.com/d/diseases_contagious_from_droplets/basics.htm|archive-date=16 July 2015}}</ref> and [[coronaviruses]].<ref name="WHO">{{cite web |title=Pass the message: Five steps to kicking out coronavirus |url=https://www.who.int/news-room/detail/23-03-2020-pass-the-message-five-steps-to-kicking-out-coronavirus |website=www.who.int |access-date=24 March 2020 |language=en}}</ref> Spread of respiratory droplets from the wearer can be reduced through wearing of a [[surgical mask]].<ref name="canada"/>

===Direct contact===
{{Further|Contagious disease}}
Direct contact occurs through skin-to-skin contact, kissing, and sexual intercourse. Direct contact also refers to contact with soil or vegetation harboring infectious organisms.<ref name=":1">{{Cite web|date=2019-02-18|title=Principles of Epidemiology: Chain of Infection|url=https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section10.html|access-date=2020-07-21|website=U.S. Centers for Disease Control and Infection|language=en-us}} {{PD-inline}}</ref>  Additionally, while fecal–oral transmission is primarily considered an indirect contact route, direct contact can also result in transmission through feces.<ref name=":2">{{Cite web| vauthors = LaMorte WW |date=2016-01-06|title=Common Vehicle Spread|url=https://sphweb.bumc.bu.edu/otlt/MPH-Modules/PH/PH709_Transmission/PH709_Transmission5.html|access-date=2020-07-21|website=Boston University School of Public Health}}</ref><ref name=":3">{{cite encyclopedia | vauthors = Whittier CA |title=Fecal-Oral Transmission|date=2017-04-16|encyclopedia=The International Encyclopedia of Primatology|pages=1| veditors = Bezanson M, MacKinnon KC, Riley E, Campbell CJ |place=Hoboken, NJ, USA|publisher=John Wiley & Sons, Inc.|language=en|doi=10.1002/9781119179313.wbprim0193|isbn=978-1-119-17931-3 }}</ref>

Diseases that can be transmitted by direct contact are called contagious (contagious is not the same as infectious; although all [[contagious diseases]] are infectious, not all infectious diseases are contagious). These diseases can also be transmitted by sharing a towel (where the towel is rubbed vigorously on both bodies) or items of clothing in close contact with the body (socks, for example) if they are not washed thoroughly between uses. For this reason, contagious diseases often break out in schools, where towels are shared and personal items of clothing accidentally swapped in the changing rooms.{{cn|date=June 2021}}

Some diseases that are transmissible by direct contact include [[athlete's foot]], [[impetigo]], syphilis, [[wart]]s, and [[conjunctivitis]].<ref>{{cite book |title=Jawetz, Melnick & Adelberg's Medical Microbiology |date=2019 |location=New York | publisher = McGraw-Hill Education |isbn=978-1-260-01202-6 |edition=28th| vauthors = Morse SA, Mietzner TA, Miller S, Riedel S }}</ref>

====Sexual====
{{main|Sexually transmitted infection}}
This refers to any infection that can be caught during sexual activity with another person, including [[sexual intercourse|vaginal]] or [[anal sex]], less commonly through [[oral sex]] (see below) and rarely through [[Non-penetrative sex#Manual sex|manual sex]] (see below). Transmission is either directly between surfaces in contact during intercourse (the usual route for [[bacterium|bacteria]]l infections and those infections causing sores) or from secretions ([[semen]] or the fluid secreted by the excited female) which carry infectious agents that get into the partner's blood stream through tiny tears in the [[Human penis|penis]], [[vagina]] or [[rectum]] (this is a more usual route for [[virus]]es). In this second case, anal sex is considerably more hazardous since the penis opens more tears in the rectum than the vagina, as the vagina is more elastic and more accommodating.{{citation needed|date=September 2015}}

Some infections transmissible by the sexual route include [[AIDS|HIV/AIDS]], [[Chlamydia infection|chlamydia]], [[genital warts]], [[gonorrhea]], [[hepatitis B]], [[syphilis]], [[herpes]], and [[trichomoniasis]].{{cn|date=June 2021}}

====Oral sex====
[[Sexually transmitted infection]]s such as HIV and hepatitis B are thought to not normally be transmitted through mouth-to-mouth contact, although it is possible to transmit some STIs between the genitals and the mouth, during oral sex. In the case of HIV, this possibility has been established. It is also responsible for the increased incidence of [[herpes simplex virus]] 1 (which is usually responsible for oral infections) in genital infections and the increased incidence of the type 2 virus (more common genitally) in oral infections.{{citation needed|date=September 2015}}

====Manual sex====
While rare in regards to this sexual practice, some infections that can spread via manual sex include [[Human papillomavirus infection|HPV]], chlamydia, and syphilis.<ref>{{cite book| last1 = Hoyle | first1 = Alice | last2 = McGeeney | first2 = Ester |title=Great Relationships and Sex Education|publisher=Taylor and Francis|year=2019|access-date=July 11, 2023|isbn=978-1-35118-825-8|url=https://books.google.com/books?id=KE7ADwAAQBAJ&pg=PT261}}</ref>

====Oral====
Infections that are transmitted primarily by oral means may be caught through direct oral contact such as [[kiss]]ing, or by indirect contact such as by sharing a drinking glass or a cigarette. Infections that are known to be transmissible by kissing or by other direct or indirect oral contact include all of the infections transmissible by droplet contact and (at least) all forms of [[herpesviridae|herpes viruses]], namely [[Cytomegalovirus]] infections herpes simplex virus (especially HSV-1) and [[infectious mononucleosis]]. {{citation needed|date=September 2015}}

====Mother-to-child transmission====
[[File:Brocky, Karoly - Mother and Child (1846-50).jpg|thumb|Brocky, Karoly - Mother and Child (1846-50)]]
{{Main|Vertically transmitted disease}}
This is from mother to child (more rarely father to child), often ''in utero'', during [[childbirth]] (also referred to as [[perinatal infection]]) or during postnatal physical contact between parents and offspring. In mammals, including humans, it occurs also via [[breastfeeding|breast milk]] (transmammary transmission). [[Infectious disease]]s that can be transmitted in this way include: HIV, hepatitis B and syphilis. Many mutualistic organisms are transmitted vertically.<ref name="auto">{{cite journal| vauthors = Ebert D |date=2013|title=The Epidemiology and Evolution of Symbionts with Mixed-Mode Transmission|journal=Annual Review of Ecology, Evolution, and Systematics|volume=44|pages=623–643|doi=10.1146/annurev-ecolsys-032513-100555}}</ref>

====Iatrogenic====
Transmission [[iatrogenic|due to medical procedures]], such as touching a wound, the use of contaminated medical equipment, or an [[Injection (medicine)|injection]] or [[organ transplant|transplantation]] of infected material. Some diseases that can be transmitted [[iatrogenic]]ally include [[Creutzfeldt–Jakob disease]], [[HIV]], and many more.<ref>{{Cite web |date=2017-10-23 |title=Creutzfeldt-Jakob disease |url=https://www.nhs.uk/conditions/creutzfeldt-jakob-disease-cjd/ |access-date=2023-09-12 |website=nhs.uk |language=en}}</ref><ref>{{cite journal | vauthors = Rouet F, Nouhin J, Zheng DP, Roche B, Black A, Prak S, Leoz M, Gaudy-Graffin C, Ferradini L, Mom C, Mam S, Gautier C, Lesage G, Ken S, Phon K, Kerleguer A, Yang C, Killam W, Fujita M, Mean C, Fontenille D, Barin F, Plantier JC, Bedford T, Ramos A, Saphonn V | display-authors = 6 | title = Massive Iatrogenic Outbreak of Human Immunodeficiency Virus Type 1 in Rural Cambodia, 2014-2015 | journal = Clinical Infectious Diseases | volume = 66 | issue = 11 | pages = 1733–1741 | date = May 2018 | pmid = 29211835 | pmc = 5963970 | doi = 10.1093/cid/cix1071 }}</ref>

==== Needle sharing ====
This is the practice of intravenous drug-users by which a needle or [[syringe]] is shared by multiple individuals to administer [[intravenous drug]]s such as heroin, steroids, and hormones. This can act as a vector for [[blood-borne disease]]s, such as [[Hepatitis C]] (HCV) and [[HIV]].<ref>{{Cite web |date=2022-03-30 |title=HIV and Injection Drug Use {{!}} HIV Transmission {{!}} HIV Basics {{!}} HIV/AIDS {{!}} CDC |url=https://www.cdc.gov/hiv/basics/hiv-transmission/injection-drug-use.html |access-date=2023-09-12 |website=www.cdc.gov |language=en-us}}</ref>

=== Indirect contact ===
Indirect contact transmission, also known as vehicle-borne transmission, involves transmission through [[contamination]] of inanimate objects.  Vehicles that may indirectly transmit an infectious agent include food, water, biologic products such as [[blood]], and [[fomite]]s such as handkerchiefs, bedding, or surgical scalpels. A vehicle may passively carry a pathogen, as in the case of food or water may carrying [[hepatitis A]] virus. Alternatively, the vehicle may provide an environment in which the agent grows, multiplies, or produces toxin, such as improperly canned foods provide an environment that supports production of [[botulinum toxin]] by ''[[Clostridium botulinum]]''.<ref name=":1" />

===Transmission by other organisms===
{{Further|Vector (epidemiology)}}
A ''[[Vector (epidemiology)|vector]]'' is an [[organism]] that does not cause disease itself but that transmits [[infection]] by conveying [[pathogen]]s from one [[Host (biology)|host]] to another.<ref>{{cite web | url = http://www.metapathogen.com | title = Pathogens and vectors | archive-url = https://web.archive.org/web/20150124020044/http://www.metapathogen.com/ | archive-date=24 January 2015 | work = MetaPathogen.com }}</ref>

Vectors may be mechanical or biological. A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner. An example of a mechanical vector is a [[housefly]], which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. The pathogen never enters the body of the fly. In contrast, biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite. Biological vectors are often responsible for serious [[blood-borne disease]]s, such as [[malaria]], [[viral encephalitis]], [[Chagas disease]], [[Lyme disease]] and [[African trypanosomiasis|African sleeping sickness]]. Biological vectors are usually, though not exclusively, [[arthropod]]s, such as [[mosquito]]es, [[tick]]s, [[flea]]s and [[Louse|lice]]. Vectors are often required in the life cycle of a pathogen. A common strategy used to control vector-borne infectious diseases is to interrupt the life cycle of a pathogen by killing the vector.{{citation needed|date=September 2015}}

===Fecal–oral===
[[File:WPA Outhouse.jpg|thumb|1940 US WPA poster encouraging modernized privies]]
{{Main|Fecal–oral route}}
In the fecal-oral route, [[pathogens]] in [[Human feces|fecal]] particles pass from one person to the mouth of another person.  Although it is usually discussed as a route of transmission, it is actually a specification of the entry and exit portals of the pathogen, and can operate across several of the other routes of transmission.<ref name=":1" />  Fecal–oral transmission is primarily considered as an indirect contact route through contaminated food or water.  However, it can also operate through direct contact with feces or contaminated body parts, such as through [[anal sex]].<ref name=":2" /><ref name=":3" />  It can also operate through droplet or airborne transmission through the [[toilet plume]] from contaminated toilets.<ref name="Mead2013">{{cite journal | vauthors = Johnson DL, Mead KR, Lynch RA, Hirst DV | title = Lifting the lid on toilet plume aerosol: a literature review with suggestions for future research | journal = American Journal of Infection Control | volume = 41 | issue = 3 | pages = 254–258 | date = March 2013 | pmid = 23040490 | pmc = 4692156 | doi = 10.1016/j.ajic.2012.04.330 }}</ref><ref name="Jon2015">{{cite journal | vauthors = Jones RM, Brosseau LM | title = Aerosol transmission of infectious disease | journal = Journal of Occupational and Environmental Medicine | volume = 57 | issue = 5 | pages = 501–508 | date = May 2015 | pmid = 25816216 | doi = 10.1097/JOM.0000000000000448 | s2cid = 11166016 }}</ref>

Main causes of fecal–oral disease transmission include lack of adequate [[sanitation]] and poor [[hygiene]] practices - which can take various forms. Fecal oral transmission can be via foodstuffs or water that has become contaminated. This can happen when people do not adequately [[Hand washing|wash their hands]] after using the [[toilet]] and before preparing food or tending to patients.{{cn|date=June 2021}}

The fecal-oral route of transmission can be a public health risk for people in [[developing countries]] who live in urban [[slum]]s without access to adequate sanitation. Here, [[Human excreta|excreta]] or untreated [[sewage]] can pollute drinking water sources ([[Groundwater pollution|groundwater]] or surface water). The people who drink the [[Water pollution|polluted water]] can become infected. Another problem in some developing countries, is [[open defecation]] which leads to disease transmission via the fecal-oral route.{{cn|date=June 2021}}

Even in developed countries there are periodic system failures resulting in a [[sanitary sewer overflow]].  This is the typical mode of transmission for infectious agents such as [[cholera]], [[hepatitis A]], [[polio]], [[Rotavirus]], ''[[Salmonella]]'', and [[parasite]]s (e.g. ''[[Ascaris lumbricoides]]'').{{cn|date=June 2021}}

==Tracking==
{{See also|Mathematical modelling of infectious disease}}
Tracking the transmission of infectious diseases is called [[disease surveillance]]. Surveillance of infectious diseases in the public realm traditionally has been the responsibility of [[public health]] agencies, on an international, national, or local level. Public health staff relies on health care workers and microbiology laboratories to report cases of [[reportable disease]]s to them. The analysis of [[aggregate data]] can show the spread of a disease and is at the core of the specialty of [[epidemiology]].
To understand the spread of the vast majority of non-notifiable diseases, data either need to be collected in a particular study, or existing data collections can be mined, such as insurance company data or antimicrobial drug sales for example.{{cn|date=June 2021}}

For diseases transmitted within an institution, such as a hospital, prison, nursing home, boarding school, orphanage, refugee camp, etc., [[infection control]] specialists are employed, who will review medical records to analyze transmission as part of a hospital epidemiology program, for example.{{cn|date=June 2021}}

Because these traditional methods are slow, time-consuming, and labor-intensive, [[proxy (statistics)|proxies]] of transmission have been sought. One proxy in the case of influenza is tracking of [[influenza-like illness]] at certain sentinel sites of health care practitioners within a state, for example.<ref>{{cite journal | vauthors = Polgreen PM, Chen Z, Segre AM, Harris ML, Pentella MA, Rushton G | title = Optimizing influenza sentinel surveillance at the state level | journal = American Journal of Epidemiology | volume = 170 | issue = 10 | pages = 1300–1306 | date = November 2009 | pmid = 19822570 | pmc = 2800268 | doi = 10.1093/aje/kwp270 }}</ref> Tools have been developed to help track influenza [[epidemic]]s by finding patterns in certain [[web search query]] activity. It was found that the frequency of influenza-related web searches as a whole rises as the number of people sick with influenza rises. Examining space-time relationships of web queries has been shown to approximate the spread of influenza<ref name=gins>{{cite journal | vauthors = Ginsberg J, Mohebbi MH, Patel RS, Brammer L, Smolinski MS, Brilliant L | title = Detecting influenza epidemics using search engine query data | journal = Nature | volume = 457 | issue = 7232 | pages = 1012–1014 | date = February 2009 | pmid = 19020500 | doi = 10.1038/nature07634 | bibcode = 2009Natur.457.1012G | url = http://li.mit.edu/Stuff/CNSE/Paper/Ginsberg09Mohebbi.pdf | url-status = dead | s2cid = 125775 | archive-url = https://web.archive.org/web/20181024023011/http://li.mit.edu/Stuff/CNSE/Paper/Ginsberg09Mohebbi.pdf | archive-date = 2018-10-24 }}</ref> and [[dengue]].<ref name=chan>{{cite journal | vauthors = Chan EH, Sahai V, Conrad C, Brownstein JS | title = Using web search query data to monitor dengue epidemics: a new model for neglected tropical disease surveillance | journal = PLOS Neglected Tropical Diseases | volume = 5 | issue = 5 | pages = e1206 | date = May 2011 | pmid = 21647308 | pmc = 3104029 | doi = 10.1371/journal.pntd.0001206 | doi-access = free }}</ref>

[[Computer simulation]]s of infectious disease spread have been used.<ref name=math>{{cite journal | vauthors = Siettos CI, Russo L | title = Mathematical modeling of infectious disease dynamics | journal = Virulence | volume = 4 | issue = 4 | pages = 295–306 | date = May 2013 | pmid = 23552814 | pmc = 3710332 | doi = 10.4161/viru.24041 }}</ref>
Human aggregation can drive transmission, [[seasonal variation]] and [[epidemic|outbreaks]] of infectious diseases, such as the annual start of school, bootcamp, the annual [[Hajj]] etc. Most recently, data from cell phones have been shown to be able to capture population movements well enough to predict the transmission of certain infectious diseases, like rubella.<ref name=pnas>{{cite journal | vauthors = Wesolowski A, Metcalf CJ, Eagle N, Kombich J, Grenfell BT, Bjørnstad ON, Lessler J, Tatem AJ, Buckee CO | display-authors = 6 | title = Quantifying seasonal population fluxes driving rubella transmission dynamics using mobile phone data | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 35 | pages = 11114–11119 | date = September 2015 | pmid = 26283349 | pmc = 4568255 | doi = 10.1073/pnas.1423542112 | doi-access = free | bibcode = 2015PNAS..11211114W }}</ref>

==Relationship with virulence and survival==

Pathogens must have a way to be transmitted from one host to another to ensure their species' survival. Infectious agents are generally specialized for a particular method of transmission. Taking an example from the respiratory route, from an evolutionary perspective viruses or bacteria that cause their host to develop coughing and sneezing symptoms have a great survival advantage, as they are much more likely to be ejected from one host and carried to another.  This is also the reason that many microorganisms cause [[diarrhea]].{{cn|date=June 2021}}

The relationship between [[virulence]] and transmission is complex and has important consequences for the long term evolution of a pathogen. Since it takes many generations for a microbe and a new host species to co-evolve, an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. If a disease is rapidly fatal, the host may die before the microbe can be passed along to another host. However, this cost may be overwhelmed by the short-term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhea aids the bacterium in finding new hosts) or many respiratory infections (sneezing and coughing create infectious [[aerosol]]s).{{cn|date=June 2021}}

Anything that reduces the rate of transmission of an infection carries [[externality|positive externalities]], which are benefits to society that are not reflected in a price to a consumer.  This is recognized implicitly when vaccines are offered for free or at a cost to the patient less than the purchase price.<ref>{{cite journal | vauthors = Graves S, Samuelson DA | title = Externalities, public goods, and infectious diseases. | journal = Real-World Economics Review | date = March 2022 | volume = 99 | pages = 25–56 | url = http://www.paecon.net/PAEReview/issue99/GravesSamuelson99.pdf }}</ref>

==Beneficial microorganisms==

The mode of transmission is also an important aspect of the biology of beneficial microbial symbionts, such as [[coral#Intracellular symbionts|coral]]-associated dinoflagellates or [[human microbiota]]. Organisms can form symbioses with microbes transmitted from their parents, from the environment or unrelated individuals, or both.{{cn|date=June 2021}}

===Vertical transmission===

[[Vertical transmission (symbiont)|Vertical transmission]] refers to acquisition of symbionts from parents (usually mothers). Vertical transmission can be intracellular (e.g. transovarial), or extracellular (for example through post-embryonic contact between parents and offspring). Both intracellular and extracellular vertical transmission can be considered a form of non-genetic [[Hologenome theory of evolution|inheritance]] or [[maternal effect|parental effect]]. It has been argued that most organisms experience some form of vertical transmission of symbionts.<ref>{{cite journal | vauthors = Funkhouser LJ, Bordenstein SR | title = Mom knows best: the universality of maternal microbial transmission | journal = PLOS Biology | volume = 11 | issue = 8 | pages = e1001631 | date = 2013 | pmid = 23976878 | pmc = 3747981 | doi = 10.1371/journal.pbio.1001631 | doi-access = free }}</ref> Canonical examples of vertically transmitted symbionts include the nutritional symbiont [[Buchnera (bacterium)|Buchnera]] in aphids (transovarially transmitted intracellular symbiont) and some components of the [[Gut flora#Acquisition of gut flora in human infants|human microbiota]] (transmitted during passage of infants through the birth canal and also through breastfeeding).{{cn|date=June 2021}}

===Horizontal transmission===

Some beneficial symbionts are acquired [[Horizontal transmission|horizontally]], from the environment or unrelated individuals. This requires that host and symbiont have some method of recognizing each other or each other's products or services. Often, horizontally acquired symbionts are relevant to [[secondary metabolism|secondary]] rather than primary metabolism, for example for use in defense against pathogens,<ref>{{cite journal | vauthors = Kaltenpoth M, Engl T |title=Defensive microbial symbionts in Hymenoptera |journal=Functional Ecology |date=2013 |volume=28 |issue=2 |doi=10.1111/1365-2435.12089 |pages=315–327|hdl=11858/00-001M-0000-000E-B76B-E |hdl-access=free }}</ref> but some primary nutritional symbionts are also horizontally (environmentally) acquired.<ref>{{cite journal | vauthors = Nussbaumer AD, Fisher CR, Bright M | title = Horizontal endosymbiont transmission in hydrothermal vent tubeworms | journal = Nature | volume = 441 | issue = 7091 | pages = 345–348 | date = May 2006 | pmid = 16710420 | doi = 10.1038/nature04793 | s2cid = 18356960 | bibcode = 2006Natur.441..345N }}</ref> Additional examples of horizontally transmitted beneficial symbionts include  [[Bobtail squid#light organ|bioluminescent bacteria associated with bobtail squid]] and [[Rhizobia|nitrogen-fixing bacteria in plants]].{{cn|date=February 2023}}

===Mixed-mode transmission===

Many microbial symbionts, including [[Gut flora#Acquisition of gut flora in human infants|human microbiota]], can be transmitted both vertically and horizontally. Mixed-mode transmission can allow symbionts to have the "best of both worlds" – they can vertically infect host offspring when host density is low, and horizontally infect diverse additional hosts when a number of additional hosts are available. Mixed-mode transmission make the outcome (degree of harm or benefit) of the relationship more difficult to predict, because the [[Fitness (biology)|evolutionary success]] of the symbiont is sometimes but not always tied to the success of the host.<ref name="auto"/>

== See also ==
* [[Bioaerosol]]
* [[Bugchasing]]
* [[Cross-species transmission]]
* [[Infectious disease#Transmission|Infectious disease: Transmission]]
* [[Rodentology]]
* [[Transmission coefficient (epidemiology)]]

== References ==
{{Reflist|30em}}

{{Concepts in infectious disease}}
{{Authority control}}

[[Category:Disease transmission| ]]
[[Category:Infectious diseases]]