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that causes .
A disease is a particular abnormal condition, a disorder of a structure or function, that affects part or all of an . The study of disease is called
which includes the causal study of . Disease is often construed as a medical condition associated with specific
and . It may be caused by external factors such as , or it may be caused by internal dysfunctions particularly of the
such as an , or a
In humans, disease is often used more broadly to refer to any condition that causes , , , , or
to the person afflicted, or similar problems for those in contact with the person. In this broader sense, it sometimes includes , , , , , isolated , deviant , and atypical
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 emotionally, as contracting and living with a disease can alter the affected person's perspective on life.
Death due to disease is called . There are four main types of disease: , deficiency diseases,
both ( and non-hereditary), and physiological diseases. Diseases can also be classified as
and . The deadliest diseases in humans are
(blood flow obstruction), followed by
In many cases, terms such as disease, disorder, morbidity and illness are used interchangeably. There are situations, however, 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 dysfunctioning of the body's normal . Commonly, the term disease is used to refer specifically to , which are clinically evident diseases that result from the presence of
microbial agents, including viruses, , fungi, protozoa, multicellular organisms, and aberrant proteins known as . An
that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal , or of a , is not considered a disease. By contrast, an infection that is asymptomatic during its , but expected to produce symptoms later, is usually considered a disease.
are all other diseases, including most forms of [], , and .
Acute disease 
disease of a short- the term sometimes also
Chronic disease 
disease that is a long-term issue
Incurable disease 
disease that cannot be cured
Terminal disease 
disease with death as an inevitable result
Illness is generally used as a synonym for disease. However, this term 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 , medical condition, such as a ), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or
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 — by the body—that helps clear infection. Such aspects of illness can include , , , , , and inability to .
In medicine, a disorder is a functional abnormality or disturbance. Medical disorders can be categorized into , , , , and . The term disorder is often considered more value-neutral and less stigmatizing than the terms disease or illness, and therefore is a preferred terminology in some circumstances. In mental health, the term mental disorder is used as a way of acknowledging the complex interaction of
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 .
Medical condition
A medical condition is a broad term that includes all diseases, , disorders, or condition that normally receives medical treatment, such as . 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
(DSM), the widely used psychiatric manual that defines all , uses the term general medical condition to refer to all diseases, illnesses, and injuries except for . This usage is also commonly seen in the psychiatric literature. Some
policies also define a medical condition as any illness, injury, or disease except for psychiatric illnesses.
As it is more
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 .
The term medical condition is also a synonym for , 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.
Morbidity (from
morbidus, meaning "sick, unhealthy") is a diseased state, , or poor health due to any cause. The term may be used to 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 .
is the simultaneous presence of two or more medical conditions, such as
and , the term "morbidity rate" can refer to either the
rate, or the
of a disease or medical condition. This measure of sickness is contrasted with the
of a condition, which is the proportion of people dying during a given time interval.
is the association of several , , and or other characteristics that often occur together. Some syndromes, such as , others, such as , have multiple possible causes. In other cases, the cause of the syndrome 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.
Predisease
Predisease is a
vanguard of a disease state.
are common examples. The nosology or epistemology of predisease is contentious, though, because there is seldom a
differentiating a legitimate concern for subclinical/prodromal/premonitory status (on one hand) and –driven
(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 , such as taking drugs that only help people with severe disease or paying for drug prescription instances whose
is minuscule (placing it in the waste category of 's "waste, fraud, and abuse" classification).
is a broad, generic label for a category of illnesses that may include affective or
instability, behavioral dysregulation, cognitive dysfunction or impairment. Specific illnesses known as mental illnesses include , generalized , , and , 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
is used technically as a legal term.
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
"Flareup" redirects here. For the Transformers character, see .
In an infectious disease, the
is the time between infection and the appearance of symptoms. The
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 , in which the virus hides in the body in an inactive state. For example, after recovery from chickenpox, the virus may remain dormant in nerve cells for many years, and later cause
(shingles).
Acute disease
is a short-lived disease, like the .
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
and periodically . 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.
A flare-up can refer to either the recurrence of symptoms or an onset of more severe symptoms.
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.
Progressive disease
is a disease whose typical natural course is the worsening of the disease until death, serious debility, or organ failure occurs. Slowly progressiv many are also . The opposite of progressive disease is stable disease or static disease: a medical condition that exists, but does not get better or worse.
is the end of a medical condition or a treatment that is very likely to end it, while
refers to the disappearance, possibly temporarily, of symptoms. Complete remission is the best possible outcome for incurable diseases.
Clinical disease
One that has clinical consequences, i.e., the stage of the disease that produces the characteristic signs and symptoms of that disease.
is the clinical disease stage of .
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.
Localized disease
is one that affects only one part of the body, such as
Disseminated disease
has s with cancer, this is usually called .
Systemic disease
is a disease that affects the entire body, such as
Main article:
Diseases may be classified by
( by which the disease is caused), or by (s). Alternatively, diseases may be classified according to the
involved, though this is often complicated since many diseases affect more than one organ.
A chief difficulty in nosology is that diseases often cannot be defined and classified clearly, especially when etiology or pathogenesis are unknown. Thus diagnostic terms often only reflect a symptom or set of symptoms ().
Classical classification of human disease derives from observational correlation between pathological analysis and clinical syndromes. Today it is preferred to classify them by their
if it is known.
Arguably the most known classification of diseases is the 's ICD. They publish periodical updates. Currently the last publication is the
Only some diseases such as
are contagious and commonly believed infectious. The micro-organisms that cause these diseases are known as pathogens and include varieties of bacteria, viruses, protozoa and fungi.
can be transmitted, e.g. by hand-to-mouth contact with infectious material on surfaces, by bites of
of the disease, and from contaminated water or food (often via
contamination), etc. In addition, there are . In some cases,
that are not readily spread from person to person play a role, while other diseases can be prevented or ameliorated with appropriate
or other lifestyle changes.
Some diseases, such as most (but not all) forms of , , and mental disorders, are . Many non-infectious diseases have a partly or completely genetic basis (see ) and may thus be transmitted from one generation to another.
are the social conditions in which people live that determine their health. Illnesses are generally related to social, economic, political, and . Social determinants of health have been recognized by several health organizations such as the Public Health Agency of
and the World Health Organization to greatly influence collective and personal well-being. The World Health Organization's Social Determinants Council also recognizes .
When the cause of a disease is poorly understood, societies tend to mythologize the disease or use it as a
or symbol of whatever that culture considers evil. For example, until the bacterial cause of
was discovered in 1882, experts variously ascribed the disease to , a , , and overindulgence in sex, rich food, or alcohol—all the
of the time.
is any disease that is caused by pathogens and transmitted through the air.
Infectious
, also known as transmissible diseases or communicable diseases, comprise clinically evident illness (i.e., characteristic medical signs and/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
– an infection, such as
or the , that commonly spreads from one person to another – and
– an disease that can spread from one person to another, but does not necessarily spread through everyday contact.
Non-communicable
is a medical condition or disease that is non-transmissible. Non-communicable diseases cannot be spread directly from one person to another.
are examples of non-communicable diseases in humans.
or food poisoning is any illness resulting from the consumption of food contaminated with pathogenic bacteria, toxins, viruses, prions or parasites.
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.
Main article:
Many diseases and disorders can be prevented through a variety of means. These include , proper , adequate ,
Main article:
or treatments are efforts to cure or improve a disease or other health problem. In the medical field, therapy is synonymous with the word treatment. Among psychologists, the term may refer specifically to
or "talk therapy". Common treatments include , , , and . Treatments may be provided by an organized , or informally, by the patient or family members.
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 .
are a subset of treatments that reverse diseases completely or end medical problems permanently. Many diseases that cannot be completely cured are still treatable.
(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.
Treatment for
must be provided promptly, often through an
or, in less critical situations, through an
Main article:
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.
Epidemiology is considered a cornerstone methodology of public health research, and is highly regarded in
for identifying
for disease. In the study of communicable and non-communicable diseases, the work of epidemiologists ranges from
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 . Epidemiologists rely on a number of other scientific disciplines such as
(to better understand disease processes),
(the current raw information available),
(to store data and map disease patterns) and
disciplines (to better understand proximate and distal risk factors). Epidemiology can help identify causes as well as guide prevention efforts.
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 level (National hospital morbidity database (NHMD), for example), or at European scale (European Hospital Morbidity Database or HMDB) but not yet at world scale.
is the impact of a health problem in an area measured by financial cost, mortality, morbidity, or other indicators.
There are several measures used to quantify the burden imposed by diseases on people. The
(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
calculated that 932 million years of potential life were lost to premature death.
(QALY) and
(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,
cause the most loss of life, but
cause the most years lost to being sick.
Disease category
Percent of all YPLLs lost, worldwide
Percent of all DALYs lost, worldwide
Percent of all YPLLs lost, Europe
Percent of all DALYs lost, Europe
Percent of all YPLLs lost, US and Canada
Percent of all DALYs lost, US and Canada
Infectious and parasitic diseases, especially , , , , and
, especially
, principally
was a status symbol in
culture: "The Tuscan General ", attributed to , 1645. It is now generally regarded as a disease.
How a society responds to diseases is the subject of .
A condition may be considered a disease in some cultures or eras but not in others. For example,
can represent wealth and abundance, and is a status symbol in famine-prone areas and some places hard-hit by .
is considered a sign of spiritual gifts among the .
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 . A person who responds to a dreaded disease, such as , in a culturally acceptable fashion may be publicly and privately honored with higher . In return for these benefits, the sick person is obligated to seek treatment and work to become well once more. As a comparison, consider , 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
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 .
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 recognitions as diseases of
(also known as "", "", and "combat fatigue") 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
as a disease could be profound, though this classification is not yet widespread.
were people who were historically shunned because they had an infectious disease, and the term "leper" still evokes . Fear of disease can still be a widespread social phenomenon, though not all diseases evoke extreme social stigma.
Social standing and economic status affect health.
are diseases that are associated with
are diseases that are associated with high social and economic status. Which diseases are associated with which states varies according to time, place, and technology. Some diseases, such as , may be associated with both poverty (poor food choices) and affluence (long lifespans and sedentary lifestyles), through different mechanisms. The term
describes diseases associated with longevity and that are more common among older people. For example,
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.
is a way of organizing a medical experience into a coherent story that illustrates the sick individual's personal experience.
People use
to make sense of their experiences with disease. The metaphors move disease from an objective thing that exists to an
experience. The most popular metaphors draw on
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 d 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
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.
is a common metaphor for : The alcoholic is enslaved by drink, and the smoker is captive to nicotine. Some cancer patients treat the loss of their hair from
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
and . Authors in the 19th century
as a symbol and a metaphor for . Victims of 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.
, a disease whose cause is currently unknown
, severe, lifelong disabilities attributable to mental and/or physical impairments
, a disease that affects very few people
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