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#REDIRECT [[Infection]]
In [[medicine]], '''infectious disease''' or '''communicable disease''' is [[disease]] caused by a [[biology|biological]] agent such as by a [[virus]], [[bacterium]] or [[parasite]]. This is contrasted to [[physics|physical]] causes, such as [[burn (injury)|burn]]s or [[chemistry|chemical]] ones such as through [[intoxication]].
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{{R with Wikidata item}}
==Basics==
{{Authority control}}

Infectious diseases are the invasion of a [[host (biology)|host]] organism by a foreign [[replicator]], generally microorganisms, often called ''microbes,'' that are invisible to the naked eye. Microbes that cause illness are also known as ''pathogens.'' The most common pathogens are various bacteria and viruses, though a number of other microorganisms, including some kinds of [[fungi]] and [[protozoa|protozoa,]] also cause disease. [[Prions]] are borderline, and [[meme]]s would not usually be considered in this scope. An infectious disease is termed ''contagious'' if it is easily transmitted from one person to another.

An organism that a microbe infects is known as the host for that microbe. In the human host, a microorganism causes disease by either disrupting a vital body process or stimulating the immune system to mount a defensive reaction. An immune response against a pathogen, which can include a high fever, inflammation, and other damaging symptoms, can be more devastating than the direct damage caused by the microbe.

===Agents and vectors===
Infectious disease requires an ''agent'' and a ''mode of transmission'' (or ''[[Vector (biology)|vector]]''). A good example is [[malaria]], which is caused by Plasmodial [[parasite]]s, chiefly ''[[Plasmodium falciparum]]'' but does not affect humans unless the vector, the [[Anopheles]] [[mosquito]], is around to introduce the parasite into the [[human]] bloodstream.

The vector does not have to be biological. Many infectious diseases are transmitted by droplets which enter the airway (e.g. [[common cold]] and [[tuberculosis]]).

==Mortality from infectious diseases==
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 killers which caused more than 100,000 deaths in 2002 (estimated). 1993 data is also included for comparison.
{| cellpadding=3 cellspacing=0 border=1
|
|
|2002
|
|''1993''
|
|-
|
|World population
| 6.2 billion
|
| 5.5 billion
|-
|
|Total deaths from all causes
| 57 million
| 100%
| 51 million
|-
| '''Rank'''
| '''Cause of death'''
| '''Number'''
| '''Percentage of total'''
| ''Number''
| ''1993 Rank''
|-
|
| I. Communicable diseases category
| 14.9 million
| 26.0%
|-
| 1
|
:[[Lower respiratory tract infection|Lower respiratory infection]]s
| 3.9 million
| 6.8%
| 4.1 million
| 1
|-
| 2
|
:[[HIV]]/[[AIDS]]
| 2.8 million
| 4.9%
| 0.7 million
| 7
|-
| 3
|
:[[Gastroenteritis|Diarrheal disease]]s
| 1.8 million
| 3.2%
| 3.0 million
| 2
|-
| 4
|
:[[Tuberculosis]] (TB)
| 1.6 million
| 2.7%
| 2.7 million
| 3
|-
| 5
|
:[[Malaria]]
| 1.3 million
| 2.2%
| 2.0 million
| 4
|-
| 6
|
:[[Measles]]
| 0.6 million
| 1.1%
| 1.1 million
| 5
|-
| 7
|
:[[Pertussis]]
| 0.30 million
| 0.5%
| 0.36 million
| 7
|-
| 8
|
:[[Tetanus]]
| 0.21 million
| 0.4%
| 0.15 million
| 12
|-
| 9
|
:[[Meningitis]]
| 0.17 million
| 0.3%
| 0.25 million
|8
|-
| 10
|
:[[Syphilis]]
| 0.16 million
| 0.3%
| 0.19 million
| 11
|-
| 11
|
:[[Hepatitis B]]
| 0.10 million
| 0.2%
| 0.93 million
| 6
|-
|
|
:[[Tropical disease]]s (6)
| 0.13 million
| 0.2%
| 0.53 million
| 9, 10, 16, 17, 18
|}

<!-- following is indented as a footnote for the table -->
:''(Note: The other categories of death are maternal and perinatal conditions (5.2%), nutritional deficiencies (0.9%), noncommunicable conditions (58.8%), and injuries (9.1%).)''
----
The top three single agent disease killers are [[HIV]]/[[AIDS]], [[tuberculosis|TB]] and [[malaria]]. From 1993 to 2002, the death ranking for AIDS went up from 7th to 2nd and Hepatitis B went down from 6th to 11th. While the number of deaths has decreased in almost every disease, it has increased four-fold in [[AIDS]].

Lower respiratory infections, which include various [[pneumonia]]s, and diarrheal diseases are caused by many different [[virus]]es, [[bacterium|bacteria]] or [[parasite]]s.

Childhood diseases include [[pertussis]], [[poliomyelitis]], [[diphtheria]], [[measles]] and [[tetanus]]. Children also make up a large percentage of lower respiratory and diarrheal deaths.

Tropical diseases include [[Chagas disease]], [[dengue fever]], [[filariasis|lymphatic filariasis]], [[leishmaniasis]], [[onchocerciasis]], [[schistosomiasis]] and [[trypanosomiasis]].

===Historic pandemics===
A [[pandemic]] (or global [[epidemic]]) is a disease that affects people over an extensive geographical area.
* The Influenza Pandemic of 1918 or [[Spanish Flu]] killed 25 to 40 million in 1 year (about 2% of world population of 1.7 billion).
** [[Influenza]] now kills about 250,000 to 500,000 worldwide each year.
* The [[Black Death]] of 1347 to 1352 killed 25 million in Europe over 5 years (estimate of 25 to 50% of Europe, Asia, and Africa - world population was 500 million).

===New diseases and pandemics===
In some cases, a microorganism and its host live in reasonable harmony. Such is the case for many tropical viruses and the insects, monkeys, or other animals in which they have lived and reproduced for thousands or millions of years. Because the microbes and their hosts have co-evolved together, the hosts have gradually become resistant to the microorganisms. But when a microbe jumps from a long-time animal host to a human being, it may cease being a harmless parasite and—simply because it is new to the human species—become a pathogen. (See [[infection]]).

With most new infectious diseases, some human action is involved, changing the environment so that an existing [[microbe]] can take up residence in a new [[ecological niche|niche.]] Once that happens, a [[pathogen]] that had been confined to a remote habitat appears in a new or wider region, or a microbe that had infected only animals suddenly begins causing human disease.

Several human activities have led to the emergence and spread of new diseases:

:'''Encroachment on wildlife habitats.''' The construction of new villages and housing developments in rural areas brings people into contact with animals--and the microbes they harbor.
:'''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.
:'''Destroying rain forests.''' As tropical countries make use of their rain forests, building roads through forests and clearing areas for settlement or commercial ventures, people encounter insects and other animals harboring unknown microorganisms.
:'''Uncontrolled urbanization.''' The rapid growth of cities in many developing countries concentrates large numbers of people in crowded areas with poor sanitation, which foster the transmission of contagious diseases.
:'''Modern transport.''' Ships and other cargo carriers often harbor unintended "passengers," such as insects and rats, that can spread diseases to faraway destinations.
:'''High-speed globe-trotting.''' With international jet-airplane travel, people infected with a new disease can carry the disease to the far side of the world before their first symptoms appear.

The relationship between virulence and transmission is complex, and has important consequences for the long term evolution of a pathogen. If a disease is rapidly fatal, the host may die before the microbe can get 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 diarrhoea aids the bacterium in finding new hosts) or many respiratory infections (sneezing, coughing etc create infectious aerosols). Since it takes time 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.

==Diagnosis and therapy==
The field of ''infectious diseases'' also occupies itself with the [[diagnosis]] and [[Medication|therapy]] of [[infection]].

===Diagnosis===
''Diagnosis'' is initially by medical history and physical examination, and imaging (such as [[X-ray]]s), but the principal tool in infectious disease is the [[microbiological culture]]. In a culture, a [[growth medium]] is provided for a particular agent. After inoculation of a specimen of diseased fluid or tissue onto the medium, it is determined whether bacterial growth occurs. This works for a number of [[bacterium|bacteria]], for example [[Staphylococcus]] or [[Streptococcus]].

Certain agents cannot be [[microbiological culture|cultured]], for example the above-mentioned ''[[Treponema pallidum]]'' and most [[virus]]es. The first [[serology|serological]] markers were developed to diagnose [[syphilis]] (the [[Wassermann test]], later replaced by the [[VDRL]] and [[TPHA]] tests). Serology involves detecting the [[antibody|antibodies]] against an infectious agent in the patient's [[blood]]. In [[immunocompromised]] patients (e.g. [[AIDS]]), [[serology]] can be troublesome, because the antibody reaction is blunted.

A more recent development is direct detection of [[virus|viral]] proteins and/or DNA in [[blood]] or [[secretion]]s. This can be done by [[PCR]] (polymerase chain reaction), involving the amplification of viral DNA and its subsequent detection with anti-DNA probes.

====The classification of infectious disease====
One way of proving that a given disease is "infectious", is to satisfy [[Koch's postulates]] ([[Robert Koch]]), which demand that the [[infectious agent]] is identified in patients and not in controls, and that patients who contract the agent also develop the disease. These postulates were tried and tested in the discovery of [[Mycobacteria]] as the cause for [[tuberculosis]]. Often, it is not possible to meet some of the criteria, even in diseases that are quite clearly infectious. For example, ''[[Treponema pallidum]]'', the causative [[spirochete]] of [[syphilis]], cannot be [[microbiological culture|cultured]] in vitro - however the organism can be cultured in rabbit testes].

[[Epidemiology]] is another important tool used to study disease in a population. For infectious diseases it helps to determine if a disease [[outbreak]] is sporadic (occasional occurrence), [[endemic (epidemiology)|endemic]] (regular cases often occurring in a region), [[epidemic]] (an unusually high number of cases in a region), or [[pandemic]] (a global epidemic).

===Therapy===
When a [[microbiological culture|culture]] has proven to be positive, the sensitivity (or, conversely, the [[antibiotic resistance]]) of an agent can be determined by exposing it to test doses of [[antibiotic]]. This way, the [[microbiology|microbiologist]] determines how sensitive the target [[bacterium]] is to a certain antibiotic. This is usually reported as being: '''S'''ensitive, '''I'''ntermediate or '''R'''esistant. The ''[[antibiogram]]'' can then be used to determine optimal therapy for the patient. This can reduce the use of [[broad-spectrum antibiotic]]s and lead to a decrease in [[antibiotic resistance]].

====The work of an infectiologist====
[[Physician|Doctor]]s who specialise in the [[medicine|medical treatment]] of infectious disease are called ''infectiologists'' or ''infectious disease specialists''. Generally, [[infection]]s are initially diagnosed by [[primary care]] physicians or [[internal medicine]] specialists. For example, an "uncomplicated" [[pneumonia]] will generally be treated by the [[internist]] or the [[pulmology|pulmonologist]] (lung physician).

The services of the infectious disease team are called for when:
* The disease has not been definitively diagnosed after an initial workup
* The patient is [[immunocompromised]] (for example, in [[AIDS]] or after [[chemotherapy]]);
* The [[infectious agent]] is of an uncommon nature (e.g. [[tropical disease]]s);
* The disease has not responded to first line [[antibiotic]]s;
* The disease might be dangerous to other patients, and the patient might have to be isolated.

The work of the infectiologist therefore entails working with patients and doctors on one hand and [[laboratory science|laboratory scientists]] and [[immunology|immunologist]]s on the other hand.

===History===
[[Anton van Leeuwenhoek]] ([[1632]]-[[1723]]) advanced the science of microscopy, allowing easy visualization of bacteria.

[[Louis Pasteur]] proved beyond doubt that certain diseases can be caused by infectious agents, and developed a vaccine for rabies.

[[Robert Koch]], mentioned above, gave the study of infectious diseases a scientific basis by formulating [[Koch's postulates]].

[[Edward Jenner]], [[Jonas Salk]] and [[Albert Sabin]] developed successful vaccines for [[Smallpox]] and [[polio]], reducing the threat of these debilitating diseases.

[[Alexander Fleming]] discovers the world's first antibiotic Penicillin which in turn inspired the discovery of the other [[antibiotics]] available today.

[[Gerhard Domagk]] develops [[Sulphonamides]], the first broad spectrum synthetic antibacterial drugs.

==See also==
* [[Infection]]
* [[Microbiology]]
* [[List of infectious diseases]]
* [[Copenhagen Consensus]]
* [[List of publications in biology#Infectious disease|Important publications in infectious diseae]]
* [[Big killer]]

==References==
*[http://www.ppidonline.com/ Mandell's Principles and Practices of Infection Diseases] or [http://www.intl.elsevierhealth.com/catalogue/title.cfm?ISBN=0443066736 this site]
*[http://www.elsevier-international.com/catalogue/title.cfm?ISBN=0702026409 Manson's Tropical Diseases]
*[[Control of Communicable Diseases Manual]] edited by James B. Chin, APHA, 2000
*[[Encyclopedia of public health]] edited by Lester Breslow, Macmillan Reference 2002
* Table: [http://www.who.int/whr/en/ The World Health Report] - 2004 Annex Table 2 and 1995 Table 5

==External links==
* [http://www.idsociety.org IDsociety] - The Infectious Disease Society of America
* [http://www.GIDEONonline.com GIDEON] - [[GIDEON-Global Infectious Diseases and Epidemiology Network|Global Infectious Diseases and Epidemiology Network]]
* [http://infectious-diseases.jwatch.org/ Infectious Diseases Journal Watch] - Medical journal summaries and physician commentary focused on infectious disease.
* [http://www.epidemi.ca EPIDEMIca] - A weblog about infectious diseases.
* [http://www.watchtower.org/library/g/2003/5/22/article_01.htm When Insects Spread Disease], discusses problem and offers tips for prevention
* [http://saei.org Sociedad Andaluza de Enfermedades Infecciosas]

{{Medicine}}

[[Category:Infectious diseases|*]]

[[de:Infektionskrankheit]]
[[es:Enfermedad infecciosa]]
[[eu:Gaixotasun infekzioso]]
[[fr:Maladie infectieuse]]
[[ko:감염병]]
[[id:Penyakit menular]]
[[lt:Infekcinė liga]]
[[nl:Infectieziekte]]
[[ja:感染症]]
[[pl:Choroba zakaźna]]
[[pt:Doença infecciosa]]
[[ro:Boală infecţioasă]]
[[simple:Infectious disease]]
[[sk:Infekčná choroba]]
[[fi:Tartuntatauti]]
[[tr:Bulaşıcı hastalık]]
[[zh:傳染病]]

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