Editing Infection (section)

===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.