Editing Infection (section)

==Epidemiology==
{{See also|Epidemic|Pandemic}}
[[File:Infectious and parasitic diseases world map-Deaths per million persons-WHO2012.svg|upright=1.3|thumb|Deaths due to infectious and parasitic diseases per million persons in 2012: {{Div col|small=yes|colwidth=10em}}{{legend|#ffff20|28–81}}{{legend|#ffe820|82–114}}{{legend|#ffd820|115–171}}{{legend|#ffc020|172–212}}{{legend|#ffa020|213–283}}{{legend|#ff9a20|284–516}}{{legend|#f08015|517–1,193}}{{legend|#e06815|1,194–2,476}}{{legend|#d85010|2,477–3,954}}{{legend|#d02010|3,955–6,812}}{{div col end}}]]
[[File:Infectious and parasitic diseases world map - DALY - WHO2004.svg|thumb|upright=1.3|[[Disability-adjusted life year]] for infectious and parasitic diseases per 100,000 inhabitants in 2004:<ref>{{cite web |url=https://www.who.int/entity/healthinfo/global_burden_disease/gbddeathdalycountryestimates2004.xls |title=Age-standardized DALYs per 100,000 by cause, and Member State, 2004 |author=World Health Organization |author-link=World Health Organization |date=February 2009 |access-date=2020-10-05 |archive-date=2021-08-28 |archive-url=https://web.archive.org/web/20210828123901/https://www.who.int/healthinfo/global_burden_disease/gbddeathdalycountryestimates2004.xls |url-status=live }}</ref>{{image key
|{{legend|#b3b3b3|no data}}
|{{legend|#ffff65|≤250}}
|{{legend|#fff200|250–500}}
|{{legend|#ffdc00|500–1000}}
|{{legend|#ffc600|1000–2000}}
|{{legend|#ffb000|2000–3000}}
|{{legend|#ff9a00|3000–4000}}
|{{legend|#ff8400|4000–5000}}
|{{legend|#ff6e00|5000–6250}}
|{{legend|#ff5800|6250–12,500}}
|{{legend|#ff4200|12,500–25,000}}
|{{legend|#ff2c00|25,000–50,000}}
|{{legend|#cb0000|≥50,000}}
}}]]

An estimated 1,680 million people died of infectious diseases in the 20th century<ref>David McCandless, [http://www.informationisbeautiful.net/visualizations/20th-century-death '20th Century Death'] {{Webarchive|url=https://web.archive.org/web/20210917222211/http://www.informationisbeautiful.net/visualizations/20th-century-death |date=17 September 2021 }}</ref> and about 10 million in 2010.<ref>{{cite news |url=http://www.cbc.ca/news/1.2802071 |title=Could Ebola rank among the deadliest communicable diseases? |publisher=CBC News |date=20 October 2014 |access-date=21 October 2014 |archive-date=27 August 2021 |archive-url=https://web.archive.org/web/20210827185958/https://www.cbc.ca/news/1.2802071 |url-status=live }}</ref>

The [[World Health Organization]] collects information on global deaths by [[ICD|International Classification of Disease (ICD) code categories]]. The following table lists the top infectious disease by number of deaths in 2002. 1993 data is included for comparison.

{| class="wikitable sortable"
|+ '''Worldwide mortality due to infectious diseases'''<ref>{{cite web | title=The World Health Report (Annex Table 2) | url=https://www.who.int/whr/2004/annex/topic/en/annex_2_en.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.who.int/whr/2004/annex/topic/en/annex_2_en.pdf |archive-date=2022-10-09 |url-status=live | year=2004}}</ref><ref>{{cite web | url=https://www.who.int/whr/1995/en/whr95_ch1_en.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.who.int/whr/1995/en/whr95_ch1_en.pdf |archive-date=2022-10-09 |url-status=live | year=1995 | title=Table 5 }}</ref>
! Rank|| Cause of death || Deaths 2002<br /> (in millions) || Percentage of<br /> all deaths || Deaths 1993<br /> (in millions)|| 1993 Rank
|- style="background:#ffdead;"
||N/A|| style="background:#ffdead;"| All infectious diseases || style="background:#ffdead;"|14.7 || style="background:#ffdead; text-align:center;"|25.9% || style="background:#ffdead;"| 16.4 || style="background:#ffdead; text-align:center;"|32.2%
|-
| 1 ||[[Lower respiratory tract infection|Lower respiratory infections]]<ref>Lower respiratory infections include various [[pneumonia]]s, [[influenza]]s and [[acute bronchitis]].</ref>
||3.9 || style="text-align:center;"|6.9%||4.1||1
|-
| 2 |||[[HIV]]/[[AIDS]] ||2.8 || style="text-align:center;"|4.9% || 0.7||7
|-
| 3 ||[[Gastroenteritis|Diarrheal diseases]]<ref>Diarrheal diseases are caused by many different organisms, including [[cholera]], [[botulism]], and ''[[Escherichia coli|E. coli]]'' <!-- THIS APPEARS TO BE A LIST OF TWO DISEASES and ONE ORGANISM -->to name a few. See also: [[Intestinal infectious diseases]]</ref>||1.8 || style="text-align:center;"|3.2% ||3.0||2
|-
| 4 ||[[Tuberculosis]] (TB)|| 1.6 || style="text-align:center;"|2.7% ||2.7 ||3
|-
| 5 ||[[Malaria]] ||1.3 || style="text-align:center;"|2.2% ||2.0 ||4
|-
| 6 || [[Measles]] ||0.6 || style="text-align:center;"|1.1% ||1.1 ||5
|-
| 7 ||[[Pertussis]] ||0.29|| style="text-align:center;"|0.5% ||0.36 ||7
|-
| 8 ||[[Tetanus]] ||0.21 || style="text-align:center;"|0.4% ||0.15 ||12
|-
| 9 ||[[Meningitis]]||0.17 || style="text-align:center;"|0.3% ||0.25 ||8
|-
| 10 ||[[Syphilis]]||0.16 || style="text-align:center;"|0.3% ||0.19 || 11
|-
| 11 ||[[Hepatitis B]] ||0.10 || style="text-align:center;"|0.2% ||0.93 || 6
|-
| 12–17 ||[[Tropical disease]]s (6)<ref>Tropical diseases include [[Chagas disease]], [[dengue fever]], [[filariasis|lymphatic filariasis]], [[leishmaniasis]], [[onchocerciasis]], [[schistosomiasis]] and [[trypanosomiasis]].</ref> ||0.13 || style="text-align:center;"|0.2% ||0.53 ||9, 10, 16–18
|-
| style="text-align:left;" colspan=6| {{small|''Note: Other causes of death include maternal and perinatal conditions (5.2%), nutritional deficiencies (0.9%),<br /> noncommunicable conditions (58.8%), and injuries (9.1%).''}}
|}

The top three single agent/disease killers are [[HIV]]/[[AIDS]], [[tuberculosis|TB]] and [[malaria]]. While the number of deaths due to nearly every disease have decreased, deaths due to HIV/AIDS have increased fourfold. Childhood diseases include [[pertussis]], [[poliomyelitis]], [[diphtheria]], [[measles]] and [[tetanus]]. Children also make up a large percentage of lower respiratory and diarrheal deaths. In 2012, approximately 3.1 million people have died due to lower respiratory infections, making it the number 4 leading cause of death in the world.<ref>{{Cite web|title = WHO {{!}} The top 10 causes of death|url = https://www.who.int/mediacentre/factsheets/fs310/en/|publisher = WHO|access-date = 2015-09-24|archive-date = 2017-05-30|archive-url = https://web.archive.org/web/20170530121727/http://www.who.int/mediacentre/factsheets/fs310/en/|url-status = live}}</ref>

===Historic pandemics===
{{See also|List of epidemics}}
[[File:Marseille-peste-Serre.jpg|thumb|The [[Great Plague of Marseille]] in 1720 killed 100,000 people in the city and the surrounding provinces.]]
With their potential for unpredictable and explosive impacts, infectious diseases have been major actors in human [[history]].<ref name=Fauci2012>{{cite journal |vauthors=Fauci AS, Morens DM |title=The perpetual challenge of infectious diseases |journal=New England Journal of Medicine |volume=366 |issue=5 |pages=454–61 |date=2012 |pmid=22296079 |doi=10.1056/NEJMra1108296 |doi-access=free }}</ref> A [[pandemic]] (or global [[epidemic]]) is a disease that affects people over an extensive geographical area. For example:
* [[Plague of Justinian]], from 541 to 542, killed between 50% and 60% of Europe's population.<ref>[http://eee.uci.edu/clients/bjbecker/PlaguesandPeople/lecture3.html "Infectious and Epidemic Disease in History"] {{webarchive |url=https://archive.today/20120712145547/http://eee.uci.edu/clients/bjbecker/PlaguesandPeople/lecture3.html |date=July 12, 2012 }}</ref>
* The [[Black Death]] of 1347 to 1352 killed 25 million in Europe over five years. The plague reduced the old world population from an estimated 450 million to between 350 and 375 million in the 14th century.
* The introduction of [[smallpox]], measles, and [[typhus]] to the areas of Central and South America by European explorers during the 15th and 16th centuries caused pandemics among the native inhabitants. Between 1518 and 1568 disease pandemics are said to have caused the population of [[Mexico]] to fall from 20 million to 3 million.<ref name= Dobson>{{cite journal| vauthors = Dobson AP, Carter ER| year = 1996| url = http://www.erin.utoronto.ca/~w3gwynne/BIO418/Dobson1996.pdf| title = Infectious Diseases and Human Population History| journal = BioScience| volume = 46| issue = 2| doi = 10.2307/1312814| pages = 115–26| jstor = 1312814| s2cid = 89361444 | doi-access = free| access-date = 2013-11-27| archive-date = 2016-03-03| archive-url = https://web.archive.org/web/20160303231520/http://www.erin.utoronto.ca/~w3gwynne/BIO418/Dobson1996.pdf| url-status = live}}</ref>
* The first European [[influenza]] epidemic occurred between 1556 and 1560, with an estimated mortality rate of 20%.<ref name= Dobson/>
* [[Smallpox]] killed an estimated 60 million Europeans during the 18th century<ref>{{Cite web |title=NCpedia {{!}} NCpedia |url=https://www.ncpedia.org/anchor/smallpox |access-date=2023-11-17 |website=www.ncpedia.org |archive-date=2023-03-28 |archive-url=https://web.archive.org/web/20230328055149/https://www.ncpedia.org/anchor/smallpox |url-status=live }}</ref> (approximately 400,000 per year).<ref>[https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=vacc.chapter.3 Smallpox and Vaccinia]. ''National Center for Biotechnology Information.'' {{webarchive |url=https://web.archive.org/web/20090601172056/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=vacc.chapter.3 |date=June 1, 2009 }}</ref> Up to 30% of those infected, including 80% of the children under 5 years of age, died from the disease, and one-third of the survivors went blind.<ref>{{cite journal |title=Smallpox: The Triumph over the Most Terrible of the Ministers of Death|first=Nicolau|last=Barquet|date=15 October 1997 |journal=Annals of Internal Medicine|volume=127|issue=8_Part_1|pages=635–42|doi=10.7326/0003-4819-127-8_Part_1-199710150-00010|pmid=9341063|s2cid=20357515}}</ref>
* In the 19th century, [[tuberculosis]] killed an estimated one-quarter of the adult population of Europe;<ref>[https://www.cdc.gov/TB/pubs/mdrtb/default.htm Multidrug-Resistant "Tuberculosis"]. ''Centers for Disease Control and Prevention.'' {{webarchive |url=https://web.archive.org/web/20100309043219/http://www.cdc.gov/TB/pubs/mdrtb/default.htm |date=March 9, 2010 }}</ref> by 1918 one in six deaths in France were still caused by TB.
* The Influenza Pandemic of 1918 (or the [[Spanish flu]]) killed 25–50 million people (about 2% of world population of 1.7 billion).<ref>{{cite web|url=http://www.history.navy.mil/library/online/influenza_main.htm|archive-url=https://web.archive.org/web/20060220113240/http://www.history.navy.mil/library/online/influenza_main.htm|url-status=dead|archive-date=20 February 2006|title=Influenza of 1918 (Spanish Flu) and the US Navy|date=20 February 2006}}</ref> Today [[Influenza]] kills about 250,000 to 500,000 worldwide each year.

* In 2021, COVID-19 emerged as a major global health crisis, directly causing 8.7 million deaths, making it one of the leading causes of mortality worldwide.<ref>{{Cite web |title=Leading causes of death |url=https://www.who.int/data/gho/data/themes/mortality-and-global-health-estimates/ghe-leading-causes-of-death |website=World Health Organization |access-date=2024-12-30}}</ref>

===Emerging diseases===
In most cases, microorganisms live in harmony with their hosts via [[mutualism (biology)|mutual]] or [[commensalism|commensal]] interactions. Diseases can emerge when existing parasites become pathogenic or when new pathogenic parasites enter a new host.

# [[Coevolution]] between [[parasite]] and [[host (biology)|host]] can lead to hosts becoming [[Immune tolerance|resistant]] to the parasites or the parasites may evolve greater [[virulence]], leading to [[immunopathology|immunopathological disease]].
# Human activity is involved with many [[emerging infectious disease]]s, such as [[environment (ecology)|environmental change]] enabling a parasite to occupy new [[ecological niche|niches]]. When that happens, a [[pathogen]] that had been confined to a remote habitat has a wider distribution and possibly a new [[host (biology)|host organism]]. Parasites jumping from nonhuman to human hosts are known as [[zoonosis|zoonoses]]. Under disease invasion, when a parasite invades a new host species, it may become pathogenic in the new host.<ref name= Krauss>{{cite book |author=Krauss H |title=Zoonoses: Infectious Diseases Transmissible from Animals to Humans |publisher=ASM Press |location=Washington, D.C. |year=2003 |edition=3rd |isbn=978-1-55581-236-2 |author2=Weber A |author3=Appel M }}</ref>

Several human activities have led to the emergence of [[zoonotic]] human pathogens, including viruses, bacteria, protozoa, and rickettsia,<ref>{{cite journal | title=Summer buzz | author=Potter P | journal=Emerg Infect Dis |date=July 2013 | volume=19 | issue=3 | doi=10.3201/eid1907.AC1907 | page=1184| pmc=3903457 }}</ref> and spread of [[Vector (epidemiology)|vector-borne]] diseases,<ref name= Krauss /> see also [[globalization and disease]] and [[wildlife disease]]:
* Encroachment on wildlife [[habitat]]s. The construction of new villages and housing developments in rural areas force animals to live in dense populations, creating opportunities for microbes to mutate and emerge.<ref>{{cite journal |author=Peter Daszak |author2=Andrew A. Cunningham |author3=Alex D. Hyatt |title=Emerging Infectious Diseases of Wildlife – Threats to Biodiversity and Human Health |journal=Science |volume=287 |issue=5452 |date=27 January 2000 |pages=443–49 |pmid= 10642539 |doi=10.1126/science.287.5452.443 |bibcode=2000Sci...287..443D }}</ref>
* Changes in [[agriculture]]. The introduction of new crops attracts new crop pests and the microbes they carry to farming communities, exposing people to unfamiliar diseases.
* The destruction of [[rain forests]]. As countries make use of their rain forests, by building roads through forests and clearing areas for settlement or commercial ventures, people encounter insects and other animals harboring previously unknown microorganisms.
* Uncontrolled [[urbanization]]. The rapid growth of cities in many developing countries tends to concentrate large numbers of people into crowded areas with poor sanitation. These conditions foster transmission of contagious diseases.
* Modern [[transport]]. Ships and other cargo carriers often harbor unintended "passengers", that can spread diseases to faraway destinations. While with international jet-airplane travel, people infected with a disease can carry it to distant lands, or home to their families, before their first symptoms appear.