Editing Human pathogen

A '''human pathogen''' is a [[pathogen]] ([[microbe]] or [[microorganism]] such as a [[virus]], [[bacterium]], [[prion]], or [[fungus]])  that causes [[disease]] in [[human]]s.

The human physiological defense against common pathogens (such as ''[[Pneumocystis]]'') is mainly the responsibility of the  [[immune system]] with help by some of the body's normal [[Human microbiome|microbiota]]. However, if the immune system or "good" microbiota are damaged in any way (such as by [[chemotherapy]], [[HIV|human immunodeficiency virus]] (HIV), or [[antibiotic]]s being taken to kill other pathogens), pathogenic [[bacteria]] that were being held at bay can proliferate and cause harm to the host. Such cases are called [[opportunistic infection]]s.

Some pathogens (such as the bacterium ''[[Yersinia pestis]]'', which may have caused the [[Black Plague]], the ''[[Variola]]'' virus, and the [[malaria]] protozoa) have been responsible for massive numbers of casualties and have had numerous effects on affected groups. Of particular note in modern times is HIV, which is known to have infected several million humans globally, along with the [[influenza]] virus. Today, while many medical advances have been made to safeguard against infection by pathogens, through the use of [[vaccination]], [[antibiotics]], and [[fungicide]], pathogens continue to threaten human life. Social advances such as [[food safety]], [[hygiene]], and [[water treatment]] have reduced the threat from some pathogens.

==Types==

===Viral===
{{Further|Viral disease}}
Pathogenic viruses are mainly those of the families of: ''[[Adenoviridae]],  [[Picornaviridae]], [[Herpesviridae]], [[Hepadnaviridae]], [[Coronaviridae]], [[Flaviviridae]], [[Retroviridae]], [[Orthomyxoviridae]], [[Paramyxoviridae]], [[Papovaviridae]], [[Polyomavirus]], [[Poxviridae]], [[Rhabdoviridae]]'', and ''[[Togaviridae]]''. Some notable pathogenic viruses cause [[smallpox]], [[influenza]], [[mumps]], [[measles]], [[chickenpox]], [[ebola]], and rubella. Viruses typically range between 20 and 300 nanometers in length.
<ref>{{Cite web |url=https://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola/qa.htm |title=Questions and Answers about Ebola Hemorrhagic Fever |access-date=2017-09-09 |archive-url=https://web.archive.org/web/20090506104535/http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola/qa.htm |archive-date=2009-05-06 |url-status=dead | work = U.S. Centers for Disease Control and Prevention }}</ref>

This type of pathogen is not cellular, and is instead composed of either RNA ([[RNA|Ribonucleic acid]]) or DNA ([[DNA|Deoxyribonucleic acid]]) within a protein shell - the [[capsid]]. Pathogenic viruses infiltrate host cells and manipulate the organelles within the cell such as the [[Ribosome]]s, [[Golgi apparatus|Golgi Apparatus]], and [[Endoplasmic reticulum|Endoplasmic Reticulum]] in order to multiply which commonly results in the death of the host cell via cellular decay. All the viruses that were contained within the [[lipid bilayer]] of the cell membrane are then released into the [[Extracellular matrix|intercellular matrix]] to infect neighboring cells to continue the [[viral life cycle]].

[[White blood cell]]s surround and consume the virus using a mechanism known as [[phagocytosis]]<ref name="Nainu_2017">{{cite journal |last1= Nainu |first1= Firzan |last2= Shiratsuchi |first2= Akiko |last3= Nakanishi |first3= Yoshinobu| date=2017-09-28 |title=Induction of Apoptosis and Subsequent Phagocytosis of Virus-Infected Cells As an Antiviral Mechanism |journal= Frontiers in Immunology |volume=8 |issue=1220 |pages=954 |doi=10.3389/fimmu.2017.01220 |doi-access= free |pmid=29033939 |pmc= 5624992 }}</ref> (a type of [[endocytosis]])<ref name="Johnson_2002">{{cite book |last1=Alberts |first1=B. |last2=Johnson |first2=A. |last3=Lewis |first3=J.  |date=2002 |title=SMolecular Biology of the Cell. 4th edition. |url=https://www.ncbi.nlm.nih.gov/books/NBK26870/#:~:text=Phagocytosis%20is%20a%20special%20form,large%20endocytic%20vesicles%20called%20phagosomes. |location=New York, USA |publisher=Garland Science  |isbn=9780815332183}}</ref> within the [[extracellular matrix]] to reduce and fight the infection. The components within the [[white blood cell]] are responsible for destroying the virus and recycling its components for the body to use.<ref>{{Cite book |last=Melloy |first=Patricia G. |url=https://doi.org/10.1201/9781003172260 |title=Viruses and Society |date=2022-07-25 |publisher=CRC Press |isbn=978-1-003-17226-0 |location=Boca Raton}}</ref>

===Bacterial===
{{Main|Pathogenic bacteria}}
[[File:Shigella stool.jpg|thumb|276x276px|A photomicrograph of a stool that has shigella dysentery. This bacteria typically causes foodborne illness.]]
Although the vast majority of bacteria are harmless or beneficial to one's body, a few pathogenic bacteria can cause [[infectious disease]]s. The most common bacterial disease is [[tuberculosis]], caused by the bacterium ''[[Mycobacterium tuberculosis]]'', which affects  about 2 million people mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as [[pneumonia]], which can be caused by bacteria such as ''[[Streptococcus]]'' and ''[[Pseudomonas]]'', and [[foodborne illness]]es, which can be caused by bacteria such as ''[[Shigella]]'', ''[[Campylobacter]]'', and ''[[Salmonella]]''. Pathogenic bacteria also cause infections such as [[tetanus]], [[typhoid fever]], [[diphtheria]], [[syphilis]], and [[Hansen's disease]]. They typically range between 1 and 5 micrometers in length.{{citation needed|date=July 2021}}

===Fungal===
{{Main|Pathogenic fungi}}
Fungi are a eukaryotic kingdom of microbes that are usually [[saprophyte]]s, but can cause diseases in humans. Life-threatening fungal infections in humans most often occur in immunocompromised patients or vulnerable people with a weakened immune system, although fungi are common problems in the immunocompetent population as the causative agents of skin, nail, or yeast infections. Most antibiotics that function on bacterial pathogens cannot be used to treat fungal infections because fungi and their hosts both have eukaryotic cells. Most clinical fungicides belong to the [[Azole|azole group]]. The typical fungal spore size is 1-40 micrometers in length.<ref>{{cite journal | vauthors = Köhler JR, Casadevall A, Perfect J | title = The spectrum of fungi that infects humans | journal = Cold Spring Harbor Perspectives in Medicine | volume = 5 | issue = 1 | pages = a019273 | date = November 2014 | pmid = 25367975 | pmc = 4292074 | doi = 10.1101/cshperspect.a019273 }}</ref>

===Other parasites===
{{main|Human parasites}}
Protozoans are single-celled eukaryotes that feed on microorganisms and organic tissues. Considered as "one-celled animal" as they have animal like behaviors such as motility, predation, and a lack of a cell wall. Many protozoan pathogens are considered human parasites as they cause a variety of diseases such as: [[malaria]], [[amoebiasis]], [[babesiosis]],
[[giardiasis]], [[toxoplasmosis]], [[cryptosporidiosis]], [[trichomoniasis]], [[Chagas disease]], [[leishmaniasis]], [[African trypanosomiasis]] (sleeping sickness), [[Acanthamoeba keratitis|''Acanthamoeba'' keratitis]], and [[primary amoebic meningoencephalitis]] (naegleriasis).
[[File:Threadworm.jpg|thumb|250x250px|Two pinworms next to a ruler, measuring in 6 millimeters in length]]
Parasitic worms (Helminths) are macroparasites that can be seen by the naked eye. Worms live and feed in their living host, receiving nourishment and shelter while affecting the host's way of digesting nutrients. They also manipulate the host's immune system by secreting immunomodulatory products<ref>{{cite book | veditors = Jirillo E, Magrone T, Miragliotta G |title=Immune Response to Parasitic Infections |year=2014 | publisher = Bentham Science Publishers | volume = 2 |isbn=978-1-60805-989-8 |doi=10.2174/97816080598501140201|s2cid=78737083 }}</ref> which allows them to live in their host for years. Many parasitic worms are more commonly intestinal that are soil-transmitted and infect the digestive tract; other parasitic worms are found in the host's blood vessels. Parasitic worms living in the host can cause weakness and even lead to many diseases. Parasitic worms can cause many diseases to both humans and animals. [[Helminthiasis]] (worm infection), [[Ascariasis]], and [[Pinworm infection|enterobiasis]] (pinworm infection) are few that are caused by various parasitic worms.{{citation needed|date=July 2021}}

===Prionic===
{{Main|Prion}}
[[File:Variant Creutzfeldt-Jakob disease (vCJD), H&E.jpg|thumb|269x269px|Magnified 100X and stained. This photomicrograph of the brain tissue shows the presence of the prominent spongiotic changes in the cortex, with the loss of neurons in a case of a variant of Creutzfeldt-Jakob disease (vCJD)]]
[[Prions]] are misfolded proteins that are transmissible and can influence abnormal folding of normal proteins in the brain. They do not contain any DNA or RNA and cannot replicate other than to convert already existing normal proteins to the misfolded state. These abnormally folded proteins are found characteristically in many neurodegenerative diseases as they aggregate the central nervous system and create plaques that damages the tissue structure. This essentially creates "holes" in the tissue. It has been found that prions transmit three ways: obtained, familial, and sporadic. It has also been found that plants play the role of vector for prions. There are eight different diseases that affect mammals that are caused by prions such as [[scrapie]], [[bovine spongiform encephalopathy]] (mad cow disease) and [[Feline spongiform encephalopathy|Feline spongiform encephalopathy (FSE)]]. There are also ten diseases that affect humans such as, [[Creutzfeldt–Jakob disease]] (CJD).<ref>{{cite journal | url = http://www.mad-cow.org/~tom/prionSP.html | archive-url = https://web.archive.org/web/20120125074734/http://www.mad-cow.org/~tom/prionSP.html | archive-date = 25 January 2012 | title = The prion diseases | vauthors = Prusiner SB | date = January 1995 | journal = Scientific American | volume = 272 | issue = 1 | pages = 48–51, 54–7 | doi = 10.1038/scientificamerican0195-48 | pmid = 7824915 | bibcode = 1995SciAm.272a..48P | url-access = subscription }}</ref> and [[Fatal familial insomnia]] (FFI).

===Animal pathogens===
Animal pathogens are disease-causing agents of wild and domestic animal species, at times including humans.<ref>{{cite book | vauthors = Hickling GJ | chapter = Pathogens, animal | veditors =  Simberloff D, Rejmánek M|title=Encyclopedia of biological invasions |date=2011 |publisher=University of California Press |location=Berkeley |isbn=978-0-520-26421-2 | quote = Animal pathogens are disease-causing agents of wild and domestic animal species, at times including humans. In the context of invasion biology, the term usually .. }}</ref>

==Virulence==
[[Virulence]] (the tendency of a pathogen to cause damage to a host's fitness) evolves when that pathogen can spread from a diseased host, despite that host being very debilitated.  An example is the malaria parasite, which can spread from a person near death, by hitching a ride to a healthy person on a mosquito that has bitten the diseased person.  This is called [[horizontal transmission]] in contrast to [[vertical transmission]], which tends to evolve symbiosis (after a period of high morbidity and mortality in the population) by linking the pathogen's evolutionary success to the evolutionary success of the host organism.

[[Evolutionary medicine]] has found that under horizontal transmission, [[Optimal virulence|the host population might never develop tolerance to the pathogen]].

==Transmission==
{{main|Transmission (medicine)}}
Transmission of pathogens occurs through many different routes, including airborne, direct or indirect contact, sexual contact, through blood, breast milk, or other body fluids, and through the fecal-oral route. One of the primary pathways by which food or water become contaminated is from the release of untreated sewage into a [[drinking water]] supply or onto cropland, with the result that people who eat or drink contaminated sources become infected. In [[developing countries]], most sewage is discharged into the environment or on cropland; even in [[developed countries]], some locations have periodic system failures that result in [[sanitary sewer overflow]]s.<ref>{{Cite journal |last1=Hrudey |first1=Steve E. |last2=Hrudey |first2=Elizabeth J. |last3=Pollard |first3=Simon J. T. |date=2006-12-01 |title=Risk management for assuring safe drinking water |url=https://www.sciencedirect.com/science/article/pii/S0160412006000821 |journal=Environment International |language=en |volume=32 |issue=8 |pages=948–957 |doi=10.1016/j.envint.2006.06.004 |pmid=16839605 |bibcode=2006EnInt..32..948H |hdl=1826/1519 |issn=0160-4120|hdl-access=free }}</ref>

==Examples==
* ''[[Bacillus anthracis]]'' — the causative agent of [[anthrax]] in humans and animals
* ''[[Clostridium botulinum]]'' — releases the most powerful [[neurotoxin]] leading to death from [[botulism]]
* ''[[Mycobacterium tuberculosis]]'' — the causative agent of most cases of [[tuberculosis]]
* ''[[Mycobacterium leprae]]'' — the bacterium that causes [[leprosy]] (Hansen's disease)
* ''[[Yersinia pestis]]'' — [[pneumonic plague|pneumonic]], [[septicemic plague|septicemic]], and the notorious [[bubonic plague|bubonic]] plagues ([[Black Death]])
* ''[[Rickettsia prowazekii]]'' — the etiologic agent of [[epidemic typhus|typhus fever]]
* ''[[Bartonella]]'' spp.
* [[Spanish influenza]] virus
* [[Entamoeba histolytica]] virus '' amoeba or amoebiasis

== See also ==
* [[Cancer bacteria]]
* [[Emerging Pathogens Institute]]
* [[Oncovirus]]
* [[List of clinically important bacteria]]
* [[Lists of diseases]]
* [[List of human diseases associated with infectious pathogens]]
* [[List of infectious diseases]]
* [[List of parasites of humans]]

== References ==
{{Reflist}}

== External links ==
* [https://web.archive.org/web/20091124082354/http://microvet.arizona.edu/Courses/MIC419/Tutorials/infectiousdisease.html Infectious Disease -- University of Arizona (microvet.arizona.edu)]
* [https://web.archive.org/web/20110429144303/http://www.d.umn.edu/~rhicks1/diversity/Pronunciation%20Guide.pdf Pronunciation Guide to Microorganisms]

[[Category:Microbiology]]