Disease: Difference between revisions

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]].

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]].

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.

: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]].

: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]].

: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.

: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.

:'''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.

:{{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:

:* benefit that outweighs the harm of any interventions taken – removing the precancerous tissue prevents cancer, and thus prevents a potential death from cancer.

: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]].

:{{See also|Exacerbation}}A flare-up can refer to either the recurrence of symptoms or an onset of more severe symptoms.

:Also called '''silent disease''', '''silent stage''', or '''asymptomatic disease'''. This is a stage in some diseases before the symptoms are first noted.

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.

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.

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.

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.

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.

[[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.

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.

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.

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.

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 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.

! 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

[[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.]]

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]].

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.

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]].

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.

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.

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.

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.