View source for Disease

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{{short description|Abnormal condition that negatively affects an organism}}
[[File:Mycobacterium tuberculosis.jpg|thumb|upright=1.3|[[Scanning electron micrograph]] of ''[[Mycobacterium tuberculosis]]'', a species of [[pathogenic bacteria]] that cause [[tuberculosis]]]]

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A '''disease''' is a particular abnormal condition that negatively affects the [[structure]] or [[function (biology)|function]] of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be '''medical conditions''' that are associated with specific [[signs and symptoms]]. A disease may be caused by external factors such as [[pathogen]]s or by internal dysfunctions. For example, internal dysfunctions of the [[immune system]] can produce a variety of different diseases, including various forms of [[immunodeficiency]], [[hypersensitivity]], [[allergy|allergies]] and [[autoimmune disorders]].

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In humans, ''disease'' is often used more broadly to refer to any condition that causes [[pain]], [[Abnormality (behavior)|dysfunction]], [[distress (medicine)|distress]], [[social problems]], or [[death]] to the person affected, or similar problems for those in contact with the person. In this broader sense, it sometimes includes [[injury|injuries]], [[disability|disabilities]], [[#Disorder|disorders]], [[syndrome]]s, [[infection]]s, isolated [[symptom]]s, deviant [[behavior]]s, and atypical [[human variability|variations]] of structure and function, while in other contexts and for other purposes these may be considered distinguishable categories. Diseases can affect people not only physically, but also mentally, as contracting and living with a disease can alter the affected person's perspective on life.

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Death due to disease is called [[death by natural causes]]. There are four main types of disease: infectious diseases, [[deficiency disease]]s, [[hereditary diseases]] (including both [[Genetic disorder|genetic diseases]] and [[Non-Mendelian inheritance|non-genetic hereditary diseases]]), and physiological diseases. Diseases can also be classified in other ways, such as [[transmission (medicine)|communicable]] versus [[non-communicable disease|non-communicable]] diseases. The deadliest diseases in humans are [[coronary artery disease]] (blood flow obstruction), followed by [[cerebrovascular disease]] and [[lower respiratory infections]]. In developed countries, the diseases that cause the most sickness overall are [[neuropsychiatric conditions]], such as [[Major depressive disorder|depression]] and [[Anxiety disorder|anxiety]].

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The study of disease is called ''[[pathology]]'', which includes the study of [[Etiology (medicine)|''etiology'']], or cause.

== Terminology == <!--T:6-->

=== Concepts === <!--T:7-->
In many cases, terms such as ''disease'', ''disorder'', ''morbidity'', ''sickness'' and ''illness'' are used interchangeably; however, there are situations when specific terms are considered preferable.

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<!-- This ";" and ":" formatting is called a "definition list". See [[Help:List#List basics]] for more information. Please do not change the formatting unless you know what you are doing. -->
;Disease
:The term ''disease'' broadly refers to any condition that impairs the normal functioning of the body. For this reason, diseases are associated with the dysfunction of the body's normal [[homeostasis|homeostatic processes]]. Commonly, the term is used to refer specifically to [[infectious disease]]s, which are clinically evident diseases that result from the presence of [[pathogenic]] microbial agents, including viruses, [[bacteria]], [[Fungus|fungi]], protozoa, multicellular organisms, and aberrant proteins known as [[prion]]s. An [[infection]] or [[colony (biology)|colonization]] that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal [[gut flora|bacteria and yeasts in the gut]], or of a [[passenger virus]], is not considered a disease. By contrast, an infection that is asymptomatic during its [[incubation period]], but expected to produce symptoms later, is usually considered a disease. [[Non-infectious disease]]s are all other diseases, including most forms of [[cancer]], [[heart disease]], and [[genetic disease]].
;Acquired disease
:An acquired disease is one that began at some point during one's lifetime, as opposed to disease that was already present at birth, which is [[congenital disorder|congenital disease]]. ''Acquired'' sounds like it could mean "caught via contagion", but it simply means acquired sometime after birth. It also sounds like it could imply secondary disease, but acquired disease can be primary disease.
;Acute disease
:An acute disease is one of a short-term nature ([[acute (medicine)|acute]]); the term sometimes also [[connotation|connotes]] a [[fulminant]] nature
;Chronic condition or chronic disease
:A [[chronic condition|chronic disease]] is one that persists over time, often characterized as at least six months but may also include illnesses that are expected to last for the entirety of one's natural life.
;Congenital disorder or congenital disease
: A [[congenital disorder]] is one that is present at [[birth]]. It is often a [[genetic disorder|genetic disease or disorder]] and can be [[heredity|inherited]]. It can also be the result of a [[vertically transmitted infection]] from the mother, such as [[HIV/AIDS]].
;Genetic disease
:A [[genetic disorder]] or disease is caused by one or more genetic [[mutation]]s. It is often inherited, but some mutations are random and [[wikt:de novo|de novo]].
;Hereditary or inherited disease
:A hereditary disease is a type of genetic disease caused by genetic mutations that are [[heredity|hereditary]] (and can run in families)
;Iatrogenic disease
:An [[Iatrogenesis|iatrogenic disease]] or condition is one that is caused by medical intervention, whether as a side effect of a treatment or as an inadvertent outcome.
;Idiopathic disease
:An [[idiopathic disease]] has an unknown cause or source. As medical science has advanced, many diseases with entirely unknown causes have had some aspects of their sources explained and therefore shed their idiopathic status. For example, when germs were discovered, it became known that they were a cause of infection, but particular germs and diseases had not been linked. In another example, it is known that [[autoimmunity]] is the cause of some forms of [[diabetes mellitus type 1]], even though the particular molecular pathways by which it works are not yet understood. It is also common to know certain factors are [[association (statistics)|associated]] with certain diseases; however, association and [[causality]] are two very different phenomena, as a third cause might be producing the disease, as well as an associated phenomenon.
;Incurable disease
:A disease that cannot be [[cure]]d. Incurable diseases are not necessarily [[terminal disease]]s, and sometimes a disease's symptoms can be treated sufficiently for the disease to have little or no impact on [[quality of life]].
;Primary disease
:A primary disease is a disease that is due to a [[root cause analysis|root cause]] of illness, as opposed to secondary disease, which is a [[sequela]], or [[complication (medicine)|complication]] that is caused by the primary disease. For example, a [[common cold]] is a primary disease, where [[rhinitis]] is a possible secondary disease, or [[sequela]]. A doctor must determine what primary disease, a cold or bacterial infection, is causing a patient's secondary [[rhinitis]] when deciding whether or not to prescribe antibiotics.
;Secondary disease
:A secondary disease is a disease that is a [[sequela]] or [[complication (medicine)|complication]] of a prior, causal disease, which is referred to as the primary disease or simply the underlying cause ([[root cause analysis|root cause]]). For example, a bacterial infection can be primary, wherein a healthy person is exposed to bacteria and becomes infected, or it can be secondary to a primary cause, that predisposes the body to infection. For example, a primary [[viral infection]] that weakens the [[immune system]] could lead to a secondary bacterial infection. Similarly, a primary [[burn]] that creates an open wound could provide an entry point for bacteria, and lead to a secondary bacterial infection.
;Terminal disease
:A terminal disease is one that is expected to have the inevitable result of death. Previously, AIDS was a terminal disease; it is now incurable, but can be managed indefinitely using medications.
;Illness
:The terms ''illness'' and ''sickness'' are both generally used as synonyms for ''disease''; however, the term ''illness'' is occasionally used to refer specifically to the patient's personal experience of his or her disease. In this model, it is possible for a person to have a disease without being ill (to have an objectively definable, but [[asymptomatic]], medical condition, such as a [[subclinical infection]], or to have a clinically apparent physical impairment but not feel sick or distressed by it), and to be ''ill'' without being ''diseased'' (such as when a person perceives a normal experience as a medical condition, or [[medicalization|medicalizes]] a non-disease situation in his or her life – for example, a person who feels unwell as a result of embarrassment, and who interprets those feelings as sickness rather than normal emotions). Symptoms of illness are often not directly the result of infection, but a collection of [[evolutionary medicine|evolved responses]] – [[sickness behavior]] by the body – that helps clear infection and promote recovery. Such aspects of illness can include [[lethargy]], [[depression (mood)|depression]], [[anorexia (symptom)|loss of appetite]], [[sleepiness]], [[hyperalgesia]], and inability to [[Attention|concentrate]].
;{{Visible anchor|Disorder}}
:A disorder is a functional abnormality or disturbance. Medical disorders can be categorized into [[mental disorder]]s, [[physical disorder]]s, [[genetic disorder]]s, [[emotional and behavioral disorders]], and [[functional disorder]]s. The term ''disorder'' is often considered more value-neutral and less stigmatizing than the terms ''disease'' or ''illness'', and therefore is preferred terminology in some circumstances. In mental health, the term ''mental disorder'' is used as a way of acknowledging the complex interaction of [[Biopsychosocial model|biological, social, and psychological factors]] in [[Psychiatry|psychiatric]] conditions; however, the term ''disorder'' is also used in many other areas of medicine, primarily to identify physical disorders that are not caused by infectious organisms, such as [[metabolic disorder]]s.
;Medical condition or health condition
:A '''medical condition''' or '''health condition''' is a broad concept that includes all diseases, [[lesion]]s, disorders, or nonpathologic condition that normally receives medical treatment, such as [[pregnancy]] or [[childbirth]]. While the term ''medical condition'' generally includes mental illnesses, in some contexts the term is used specifically to denote any illness, injury, or disease except for mental illnesses. The [[Diagnostic and Statistical Manual of Mental Disorders]] (DSM), the widely used psychiatric manual that defines all [[mental disorder]]s, uses the term ''general medical condition'' to refer to all diseases, illnesses, and injuries except for [[mental disorder]]s. This usage is also commonly seen in the psychiatric literature. Some [[health insurance]] policies also define a ''medical condition'' as any illness, injury, or disease except for psychiatric illnesses.
:As it is more [[value-neutral]] than terms like ''disease'', the term ''medical condition'' is sometimes preferred by people with health issues that they do not consider deleterious. On the other hand, by emphasizing the medical nature of the condition, this term is sometimes rejected, such as by proponents of the [[autism rights movement]].
:The term ''medical condition'' is also a synonym for ''[[medical state]]'', in which case it describes an individual patient's current state from a medical standpoint. This usage appears in statements that describe a patient as being ''in critical condition'', for example.
;{{Visible anchor|Morbidity}}
:'''Morbidity''' ({{ety|la|morbidus|sick, unhealthy}}) is a diseased state, [[disability]], or poor health due to any cause. The term may refer to the existence of any form of disease, or to the degree that the health condition affects the patient. Among severely ill patients, the level of morbidity is often measured by [[ICU scoring systems]]. [[Comorbidity]], or co-existing disease, is the simultaneous presence of two or more medical conditions, such as [[schizophrenia]] and [[substance abuse]].
:In [[epidemiology]] and [[actuarial science]], the term ''morbidity'' (also ''morbidity rate'' or ''morbidity frequency'') can refer to either the [[incidence (epidemiology)|incidence]] rate, the [[prevalence]] of a disease or medical condition, or the percentage of people who experience a given condition within a given timeframe (e.g., 20% of people will get [[influenza]] in a year). This measure of sickness is contrasted with the [[mortality rate]] of a condition, which is the proportion of people dying during a given time interval. Morbidity rates are used in actuarial professions, such as health insurance, life insurance, and long-term care insurance, to determine the correct premiums to charge to customers. Morbidity rates help insurers predict the likelihood that an insured will contract or develop any number of specified diseases.
; Pathosis or pathology
:''Pathosis'' (plural ''pathoses'') is synonymous with ''disease''. The word ''[[pathology]]'' also has this [[word sense|sense]], in which it is commonly used by physicians in the [[medical literature]], although [[wikt:pathology#Usage notes|some editors prefer to reserve ''pathology'' to its other senses]]. Sometimes a slight [[connotation|connotative]] shade causes preference for ''pathology'' or ''pathosis'' implying "some [as yet poorly analyzed] [[pathophysiology|pathophysiologic process]]" rather than ''disease'' implying "a specific disease entity as defined by diagnostic criteria being already met". This is hard to quantify [[denotation|denotatively]], but it explains why [[cognitive synonymy]] is not invariable.
;Syndrome
:A [[syndrome]] is the association of several [[signs and symptoms]], or other characteristics that often occur together, regardless of whether the cause is known. Some syndromes such as [[Down syndrome]] are known to have only one cause (an extra [[chromosome]] at birth). Others such as [[Parkinsonian syndrome]] are known to have multiple possible causes. [[Acute coronary syndrome]], for example, is not a single disease itself but is rather the manifestation of any of several diseases including [[myocardial infarction]] secondary to [[coronary artery disease]]. In yet other syndromes, however, the [[idiopathy|cause is unknown]]. A familiar syndrome name often remains in use even after an underlying cause has been found or when there are a number of different possible primary causes. Examples of the first-mentioned type are that [[Turner syndrome]] and [[DiGeorge syndrome]] are still often called by the "syndrome" name despite that they can also be viewed as disease entities and not solely as sets of signs and symptoms.
;Predisease
:{{anchor|Predisease}}'''Predisease''' is a [[subclinical]] or [[prodrome|prodromal]] vanguard of a disease. [[Prediabetes]] and [[prehypertension]] are common examples. The [[nosology]] or epistemology of predisease is contentious, though, because there is seldom a [[bright line]] differentiating a legitimate concern for subclinical/prodromal/premonitory status (on one hand) and [[conflict of interest]]–driven [[disease mongering]] or [[medicalization]] (on the other hand). Identifying legitimate predisease can result in useful preventive measures, such as motivating the person to get a healthy amount of physical exercise, but labeling a healthy person with an unfounded notion of predisease can result in [[overtreatment]], such as taking drugs that only help people with severe disease or paying for drug prescription instances whose [[benefit–cost ratio]] is minuscule (placing it in the waste category of [[Centers for Medicare and Medicaid Services|CMS]]' "waste, fraud, and abuse" classification). Three requirements for the legitimacy of calling a condition a predisease are:
:* a truly high risk for progression to disease – for example, a [[Precancerous condition|pre-cancer]] will almost certainly turn into cancer over time
:* actionability for risk reduction – for example, removal of the precancerous tissue prevents it from turning into a potentially deadly cancer
:* benefit that outweighs the harm of any interventions taken – removing the precancerous tissue prevents cancer, and thus prevents a potential death from cancer.

=== Types by body system === <!--T:9-->
;Mental
:[[Mental disorder|Mental illness]] is a broad, generic label for a category of illnesses that may include affective or [[emotion]]al instability, behavioral dysregulation, cognitive dysfunction or impairment. Specific illnesses known as mental illnesses include [[major depressive disorder|major depression]], generalized [[anxiety disorder]]s, [[schizophrenia]], and [[attention deficit hyperactivity disorder]], to name a few. Mental illness can be of biological (e.g., anatomical, chemical, or genetic) or psychological (e.g., trauma or conflict) origin. It can impair the affected person's ability to work or study and can harm interpersonal relationships. The term [[insanity]] is used technically as a legal term.
;Organic
:An organic disease is one caused by a physical or physiological change to some tissue or organ of the body. The term sometimes excludes infections. It is commonly used in contrast with mental disorders. It includes emotional and behavioral disorders if they are due to changes to the physical structures or functioning of the body, such as after a [[stroke]] or a traumatic brain injury, but not if they are due to [[psychosocial]] issues.

=== Stages === <!--T:10-->
{{redirect|Flareup|the Transformers character|Flareup (Transformers)}}
In an infectious disease, the [[incubation period]] is the time between infection and the appearance of symptoms. The [[latency period]] is the time between infection and the ability of the disease to spread to another person, which may precede, follow, or be simultaneous with the appearance of symptoms. Some viruses also exhibit a dormant phase, called [[viral latency]], in which the virus hides in the body in an inactive state. For example, [[varicella zoster virus]] causes [[chickenpox]] in the [[acute (medicine)|acute phase]]; after recovery from chickenpox, the virus may remain dormant in nerve cells for many years, and later cause [[herpes zoster]] (shingles).
;Acute disease
:An [[acute disease]] is a short-lived disease, like the [[common cold]].
;Chronic disease
:A [[chronic disease]] is one that lasts for a long time, usually at least six months. During that time, it may be constantly present, or it may go into [[remission (medicine)|remission]] and periodically [[relapse]]. A chronic disease may be stable (does not get any worse) or it may be progressive (gets worse over time). Some chronic diseases can be permanently cured. Most chronic diseases can be beneficially treated, even if they cannot be permanently cured.
;Clinical disease
:One that has clinical consequences; in other words, the stage of the disease that produces the characteristic signs and symptoms of that disease. [[AIDS]] is the clinical disease stage of [[HIV infection]].
;Cure
:A [[cure]] is the end of a medical condition or a treatment that is very likely to end it, while [[Remission (medicine)|remission]] refers to the disappearance, possibly temporarily, of symptoms. Complete remission is the best possible outcome for incurable diseases.
;Flare-up
:{{See also|Exacerbation}}A flare-up can refer to either the recurrence of symptoms or an onset of more severe symptoms.
;Progressive disease
:[[Progressive disease]] is a disease whose typical natural course is the worsening of the disease until death, serious debility, or organ failure occurs. Slowly progressive diseases are also [[chronic disease]]s; many are also [[degenerative disease]]s. The opposite of progressive disease is ''stable disease'' or ''static disease'': a medical condition that exists, but does not get better or worse.
;{{Visible anchor|Refractory disease}}
:A refractory disease is a disease that resists treatment, especially an individual case that resists treatment more than is normal for the specific disease in question.
;Subclinical disease
:Also called '''silent disease''', '''silent stage''', or '''asymptomatic disease'''. This is a stage in some diseases before the symptoms are first noted.
;Terminal phase
:If a person will die soon from a disease, regardless of whether that disease typically causes death, then the stage between the earlier disease process and active dying is the terminal phase.
;Recovery
:{{See also|Convalescence|Recuperation (recovery)|label 2=Recuperation}}Recovery can refer to the repairing of physical processes (tissues, organs etc.) and the resumption of healthy functioning after damage causing processes have been cured.

=== Extent === <!--T:11-->
[[File:Poison ivy rash.JPG|175px|thumb|alt=skin rash on the leg|This rash only affects one part of the body, so it is a localized disease]]
;Localized disease
:A [[localized disease]] is one that affects only one part of the body, such as [[athlete's foot]] or an [[Human eye#Eye disease|eye infection]].
;Disseminated disease
:A [[disseminated disease]] has spread to other parts; with cancer, this is usually called [[metastatic disease]].
;Systemic disease
:A [[systemic disease]] is a disease that affects the entire body, such as [[influenza]] or [[hypertension|high blood pressure]].

== Classification == <!--T:12-->
{{Main|Nosology|Medical classification}}
Diseases may be classified by cause, [[pathogenesis]] ([[mechanism (biology)|mechanism]] by which the disease is caused), or by [[symptom]](s). Alternatively, diseases may be classified according to the [[organ system]] involved, though this is often complicated since many diseases affect more than one organ.

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A chief difficulty in nosology is that diseases often cannot be defined and classified clearly, especially when cause or pathogenesis are unknown. Thus diagnostic terms often only reflect a symptom or set of symptoms ([[syndrome]]).

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Classical classification of human disease derives from the observational correlation between pathological analysis and clinical syndromes. Today it is preferred to classify them by their cause if it is known.

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The most known and used classification of diseases is the [[World Health Organization]]'s [[International Statistical Classification of Diseases and Related Health Problems|ICD]]. This is periodically updated. Currently, the last publication is the [[ICD-11]].

== Causes == <!--T:16-->
{{See also|Cause (medicine)|Transmission (medicine)}}
Only some diseases such as [[influenza]] are contagious and commonly believed infectious. The [[microorganism]]s that cause these diseases are known as pathogens and include varieties of bacteria, viruses, protozoa, and fungi. [[Infectious disease]]s can be transmitted, e.g. by hand-to-mouth contact with infectious material on surfaces, by bites of [[vector (epidemiology)|insects or other carriers]] of the disease, and from contaminated water or food (often via [[feces|fecal]] contamination), etc. Also, there are [[sexually transmitted disease]]s. In some cases, microorganisms that are not readily spread from person to person play a role, while other diseases can be prevented or ameliorated with appropriate [[nutrition]] or other lifestyle changes.

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Some diseases, such as most (but [[Infectious cancer|not all]]) forms of [[cancer]], [[heart disease]], and mental disorders, are [[non-infectious disease]]s. Many non-infectious diseases have a partly or completely genetic basis (see [[genetic disorder]]) and may thus be transmitted from one generation to another.

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[[Social determinants of health]] are the social conditions in which people live that determine their health. Illnesses are generally related to social, economic, political, and [[Environmental disease|environmental circumstances]]. Social determinants of health have been recognized by several health organizations such as the Public Health Agency of [[Canada]] and the World Health Organization to greatly influence collective and personal well-being. The World Health Organization's Social Determinants Council also recognizes [[Social determinants of health in poverty]].

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When the cause of a disease is poorly understood, societies tend to mythologize the disease or use it as a [[metaphor]] or symbol of whatever that culture considers evil. For example, until the bacterial cause of [[tuberculosis]] was discovered in 1882, experts variously ascribed the disease to [[hereditary disease|heredity]], a [[sedentary lifestyle]], [[depression (mood)|depressed mood]], and overindulgence in sex, rich food, or alcohol, all of which were [[social ills]] at the time.

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When a disease is caused by a [[pathogen]]ic organism (e.g., when [[malaria]] is caused by ''[[Plasmodium]]''), one should not confuse the [[pathogen]] (the cause of the disease) with disease itself. For example, [[West Nile virus]] (the pathogen) causes [[West Nile fever]] (the disease). The misuse of basic definitions in [[epidemiology]] is frequent in scientific publications.

=== Types of causes === <!--T:21-->
[[File:Walking the dog 7th Brigade Park Chermside P1040698.jpg|alt=A child rides a bicycle. An adult and a child walk a dog along a path in a green park..|thumb|Regular physical activity, such as riding a bicycle or walking, reduces the risk of lifestyle diseases.]]<!-- This special list formatting is designed for definitions like these. See [[Help:List]] before changing. -->
;Airborne: An [[airborne disease]] is any disease that is caused by pathogens and transmitted through the air.
;Foodborne: [[Foodborne illness]] or food poisoning is any illness resulting from the consumption of food contaminated with pathogenic bacteria, toxins, viruses, prions or parasites.
;Infectious: [[Infectious disease]]s, also known as transmissible diseases or communicable diseases, comprise clinically evident illness (i.e., characteristic medical signs or symptoms of disease) resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism. Included in this category are ''[[contagious disease]]s'' – an infection, such as [[influenza]] or the [[common cold]], that commonly spreads from one person to another – and ''[[communicable disease]]s'' – a disease that can spread from one person to another, but does not necessarily spread through everyday contact.
;Lifestyle: A [[lifestyle disease]] is any disease that appears to increase in frequency as countries become more industrialized and people live longer, especially if the risk factors include behavioral choices like a sedentary lifestyle or a diet high in unhealthful foods such as refined carbohydrates, trans fats, or alcoholic beverages.
;Non-communicable: A [[non-communicable disease]] is a medical condition or disease that is non-transmissible. Non-communicable diseases cannot be spread directly from one person to another. [[Heart disease]] and [[cancer]] are examples of non-communicable diseases in humans.

== Prevention == <!--T:22-->
{{Main|Preventive medicine}}
Many diseases and disorders can be prevented through a variety of means. These include [[sanitation]], proper [[nutrition]], adequate [[exercise]], [[vaccination]]s and other [[self-care]] and [[public health]] measures.

== Treatments == <!--T:23-->
{{Main|Therapy}}
Medical therapies or treatments are efforts to cure or improve a disease or other health problems. In the medical field, therapy is synonymous with the word ''treatment''. Among psychologists, the term may refer specifically to [[psychotherapy]] or "talk therapy". Common treatments include [[medication]]s, [[surgery]], [[medical devices]], and [[self-care]]. Treatments may be provided by an organized [[health care system]], or informally, by the patient or family members.

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[[Preventive healthcare]] is a way to avoid an injury, sickness, or disease in the first place. A treatment or cure is applied after a medical problem has already started. A treatment attempts to improve or remove a problem, but treatments may not produce permanent cures, especially in [[chronic diseases]]. [[Cure]]s are a subset of treatments that reverse diseases completely or end medical problems permanently. Many diseases that cannot be completely cured are still treatable. [[Pain management]] (also called pain medicine) is that branch of medicine employing an interdisciplinary approach to the relief of pain and improvement in the quality of life of those living with pain.

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Treatment for [[medical emergencies]] must be provided promptly, often through an [[emergency department]] or, in less critical situations, through an [[urgent care]] facility.

== Epidemiology == <!--T:26-->
{{Main|Epidemiology}}
Epidemiology is the study of the factors that cause or encourage diseases. Some diseases are more common in certain geographic areas, among people with certain genetic or socioeconomic characteristics, or at different times of the year.

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Epidemiology is considered a cornerstone methodology of public health research and is highly regarded in [[evidence-based medicine]] for identifying [[risk factor]]s for diseases. In the study of communicable and non-communicable diseases, the work of epidemiologists ranges from [[outbreak]] investigation to study design, data collection, and analysis including the development of statistical models to test hypotheses and the documentation of results for submission to peer-reviewed journals. Epidemiologists also study the interaction of diseases in a population, a condition known as a [[syndemic]]. Epidemiologists rely on a number of other scientific disciplines such as [[biology]] (to better understand disease processes), [[biostatistics]] (the current raw information available), [[Geographic Information Science]] (to store data and map disease patterns) and [[social science]] disciplines (to better understand proximate and distal risk factors). Epidemiology can help identify causes as well as guide prevention efforts.

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In studying diseases, epidemiology faces the challenge of defining them. Especially for poorly understood diseases, different groups might use significantly different definitions. Without an agreed-on definition, different researchers may report different numbers of cases and characteristics of the disease.

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Some morbidity databases are compiled with data supplied by states and territories health authorities, at national levels or larger scale (such as European Hospital Morbidity Database (HMDB)) which may contain hospital discharge data by detailed diagnosis, age and sex. The European HMDB data was submitted by European countries to the World Health Organization Regional Office for Europe.

=== Burdens of disease === <!--T:30-->
[[Disease burden]] is the impact of a health problem in an area measured by financial cost, mortality, morbidity, or other indicators.

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There are several measures used to quantify the burden imposed by diseases on people. The [[years of potential life lost]] (YPLL) is a simple estimate of the number of years that a person's life was shortened due to a disease. For example, if a person dies at the age of 65 from a disease, and would probably have lived until age 80 without that disease, then that disease has caused a loss of 15 years of potential life. YPLL measurements do not account for how disabled a person is before dying, so the measurement treats a person who dies suddenly and a person who died at the same age after decades of illness as equivalent. In 2004, the [[World Health Organization]] calculated that 932 million years of potential life were lost to premature death.

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The [[quality-adjusted life year]] (QALY) and [[disability-adjusted life year]] (DALY) metrics are similar but take into account whether the person was healthy after diagnosis. In addition to the number of years lost due to premature death, these measurements add part of the years lost to being sick. Unlike YPLL, these measurements show the burden imposed on people who are very sick, but who live a normal lifespan. A disease that has high morbidity, but low mortality, has a high DALY and a low YPLL. In 2004, the World Health Organization calculated that 1.5&nbsp;billion disability-adjusted life years were lost to disease and injury. In the developed world, [[heart disease]] and [[stroke]] cause the most loss of life, but [[neuropsychiatric conditions]] like [[major depressive disorder]] cause the most years lost to being sick.

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{| class="wikitable sortable"
|-
! Disease category
! scope="col" style="width:10%;" | Percent of all YPLLs lost, worldwide
! scope="col" style="width:10%;" | Percent of all DALYs lost, worldwide
! scope="col" style="width:10%;" | Percent of all YPLLs lost, Europe
! scope="col" style="width:10%;" | Percent of all DALYs lost, Europe
! scope="col" style="width:10%;" | Percent of all YPLLs lost, US and Canada
! scope="col" style="width:10%;" | Percent of all DALYs lost, US and Canada
|-
| Infectious and parasitic diseases, especially [[lower respiratory tract infection]]s, [[diarrhea]], [[AIDS]], [[tuberculosis]], and [[malaria]]
| 37%
| 26%
| 9%
| 6%
| 5%
| 3%
|-
| [[Neuropsychiatric conditions]], e.g. [[major depressive disorder|depression]]
| 2%
| 13%
| 3%
| 19%
| 5%
| 28%
|-
| [[Injuries]], especially [[motor vehicle accidents]]
| 14%
| 12%
| 18%
| 13%
| 18%
| 10%
|-
| [[Cardiovascular disease]]s, principally [[heart attacks]] and [[stroke]]
| 14%
| 10%
| 35%
| 23%
| 26%
| 14%
|-
| [[Premature birth]] and other [[perinatal]] deaths
| 11%
| 8%
| 4%
| 2%
| 3%
| 2%
|-
| [[Cancer]]
| 8%
| 5%
| 19%
| 11%
| 25%
| 13%
|}

== Society and culture == <!--T:34-->
[[File:Charles Mellin (attributed) - Portrait of a Gentleman - Google Art Project.jpg|thumb|upright|[[Obesity]] was a status symbol in [[Renaissance]] culture: "The Tuscan General [[Alessandro del Borro]]", attributed to [[Andrea Sacchi]], 1645. It is now generally regarded as a disease.]]

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How a society responds to diseases is the subject of [[medical sociology]].

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A condition may be considered a disease in some cultures or eras but not in others. For example, [[obesity]] can represent wealth and abundance, and is a status symbol in famine-prone areas and some places hard-hit by [[AIDS wasting|HIV/AIDS]]. [[Epilepsy]] is considered a sign of spiritual gifts among the [[Hmong people]].

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Sickness confers the social legitimization of certain benefits, such as illness benefits, work avoidance, and being looked after by others. The person who is sick takes on a social role called the [[sick role]]. A person who responds to a dreaded disease, such as [[cancer survivor|cancer]], in a culturally acceptable fashion may be publicly and privately honored with higher [[social status]]. In return for these benefits, the sick person is obligated to seek treatment and work to become well once more. As a comparison, consider [[pregnancy]], which is not interpreted as a disease or sickness, even if the mother and baby may both benefit from medical care.

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Most religions grant exceptions from religious duties to people who are sick. For example, one whose life would be endangered by [[fasting]] on [[Yom Kippur]] or during [[Ramadan]] is exempted from the requirement, or even forbidden from participating. People who are sick are also exempted from social duties. For example, ill health is the only socially acceptable reason for an American to refuse an invitation to the [[White House]].

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The identification of a condition as a disease, rather than as simply a variation of human structure or function, can have significant social or economic implications. The controversial recognition of diseases such as [[repetitive stress injury]] (RSI) and [[post-traumatic stress disorder]] (PTSD) has had a number of positive and negative effects on the financial and other responsibilities of governments, corporations, and institutions towards individuals, as well as on the individuals themselves. The social implication of viewing [[senescence|aging]] as a disease could be profound, though this classification is not yet widespread.

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[[Leprosy|Lepers]] were people who were historically shunned because they had an infectious disease, and the term "leper" still evokes [[social stigma]]. Fear of disease can still be a widespread social phenomenon, though not all diseases evoke extreme social stigma.

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Social standing and economic status affect health. [[Diseases of poverty]] are diseases that are associated with [[poverty]] and low social status; [[diseases of affluence]] are diseases that are associated with high social and economic status. Which diseases are associated with which states vary according to time, place, and technology. Some diseases, such as [[diabetes mellitus]], may be associated with both poverty (poor food choices) and affluence (long lifespans and sedentary lifestyles), through different mechanisms. The term [[lifestyle disease|''lifestyle diseases'']] describes diseases associated with longevity and that are more common among older people. For example, [[cancer]] is far more common in societies in which most members live until they reach the age of 80 than in societies in which most members die before they reach the age of 50.

=== Language of disease === <!--T:42-->
An [[illness narrative]] is a way of organizing a medical experience into a coherent story that illustrates the sick individual's personal experience.

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People use [[metaphor]]s to make sense of their experiences with disease. The metaphors move disease from an objective thing that exists to an [[affective]] experience. The most popular metaphors draw on [[military]] concepts: Disease is an enemy that must be feared, fought, battled, and routed. The patient or the healthcare provider is a [[warrior]], rather than a passive victim or bystander. The agents of communicable diseases are [[Invasion|invaders]]; non-communicable diseases constitute internal [[insurrection]] or [[civil war]]. Because the threat is urgent, perhaps a matter of life and death, unthinkably radical, even oppressive, measures are society's and the patient's moral duty as they courageously mobilize to struggle against destruction. The [[War on Cancer]] is an example of this metaphorical use of language. This language is empowering to some patients, but leaves others feeling like they are failures.

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Another class of metaphors describes the experience of illness as a journey: The person travels to or from a place of disease, and changes himself, discovers new information, or increases his experience along the way. He may travel "on the road to recovery" or make changes to "get on the right track" or choose "pathways". Some are explicitly immigration-themed: the patient has been exiled from the home territory of health to the land of the ill, changing identity and relationships in the process. This language is more common among British healthcare professionals than the language of physical aggression.

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Some metaphors are disease-specific. [[Slavery]] is a common metaphor for [[addiction]]s: The alcoholic is enslaved by drink, and the smoker is captive to nicotine. Some cancer patients treat the loss of their hair from [[chemotherapy]] as a [[metonymy]] or metaphor for all the losses caused by the disease.

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Some diseases are used as metaphors for social ills: "Cancer" is a common description for anything that is endemic and destructive in society, such as poverty, injustice, or racism. AIDS was seen as a divine judgment for moral decadence, and only by purging itself from the "pollution" of the "invader" could society become healthy again. More recently, when AIDS seemed less threatening, this type of emotive language was applied to [[Avian flu outbreak of 2009|avian flu]] and [[type 2 diabetes mellitus]]. Authors in the 19th century [[Tuberculosis in popular culture|commonly used tuberculosis]] as a symbol and a metaphor for [[Transcendence (religion)|transcendence]]. People with the disease were portrayed in literature as having risen above daily life to become ephemeral objects of spiritual or artistic achievement. In the 20th century, after its cause was better understood, the same disease became the emblem of poverty, squalor, and other social problems.

== See also == <!--T:47-->
{{Portal|Medicine|Biology}}

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{{col div|colwidth=30em}}
* [[Cryptogenic disease]], a disease whose cause is currently unknown
* [[Developmental disability]], severe, lifelong disabilities attributable to mental or physical impairments
* [[Environmental disease]]
* [[Host–pathogen interaction]]
* [[Mitochondrial disease]]
* [[Plant pathology]]
* [[Rare disease]], a disease that affects very few people
* [[Sociology of health and illness]]
* [[Syndrome]]
* [[Philosophy of medicine]]
{{colend}}

== External links == <!--T:49-->
{{Wikimedia|collapsible=true|c=Category:Disease|s=no|v=Pathology|d=Q12136}}
* [https://medlineplus.gov/healthtopics.html Health Topics], [[MedlinePlus]] descriptions of most diseases, with access to current research articles.
* [https://www.ncbi.nlm.nih.gov/omim/ OMIM] Comprehensive information on genes that cause disease at [[Online Mendelian Inheritance in Man]]
* [http://ctdbase.org/ CTD] The [[Comparative Toxicogenomics Database]] is a scientific resource connecting chemicals, genes, and human diseases.
* [https://www.nlm.nih.gov/ NLM] Comprehensive database from the [[US National Library of Medicine]]
* [https://www.cdc.gov/health-topics.html#a Health Topics A–Z], fact sheets about many common diseases at [[Centers for Disease Control]]
* [https://www.merckmanuals.com/home The Merck Manual] containing detailed description of most diseases
* [https://www.who.int/publications/i/item/9789241563710 Report: ''The global burden of disease''] from [[World Health Organization]] (WHO), 2004
* [https://siteman.wustl.edu/prevention/ydr/ Free online health-risk assessment] by [[Your Disease Risk]] at Washington University in St Louis
* [http://www.bbc.co.uk/programmes/p00548m0 "Man and Disease"], BBC Radio 4 discussion with Anne Hardy, David Bradley & Chris Dye (''In Our Time'', Dec. 15, 2002)

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{{Medical terms to describe disease conditions}}
{{Pathology}}

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{{二次利用|date=28 December 2022}}
[[Category:Diseases and disorders| ]]
[[Category:Actuarial science]]
[[Category:Medical terminology]]
[[Category:Medicine in society]]
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