Disease

For example, internal dysfunctions of the immune system can produce a variety of different diseases, including various forms of immunodeficiency, hypersensitivity, allergies and autoimmune disorders.

The immune system is a host defense system comprising many biological structures and processes within an organism that protects against disease.

Diseases are often construed as medical conditions that are associated with specific symptoms and signs. In this broader sense, it sometimes includes injuries, disabilities, disorders, syndromes, infections, isolated symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts and for other purposes these may be considered distinguishable categories.

A symptom (from Greek σύμπτωμα, "accident, misfortune, that which befalls", from συμπίπτω, "I befall", from συν- "together, with" and πίπτω, "I fall") is a departure from normal function or feeling which is noticed by a patient, reflecting the presence of an unusual state, or of a disease.

In humans, disease is often used more broadly to refer to any condition that causes pain, dysfunction, distress, social problems, or death to the person afflicted, or similar problems for those in contact with the person.

Phenomena which commonly bring about death include aging, predation, malnutrition, disease, suicide, homicide, starvation, dehydration, and accidents or major trauma resulting in terminal injury.

In this broader sense, it sometimes includes injuries, disabilities, disorders, syndromes, infections, isolated symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts and for other purposes these may be considered distinguishable categories.

A syndrome is a set of medical signs and symptoms that are correlated with each other and, often, with a particular disease or disorder.

The study of disease is called pathology, which includes the study of etiology, or cause. Pathosis (plural pathoses) is synonymous with disease. The word pathology also has this sense, in which it is commonly used by physicians in the medical literature, although some editors prefer to reserve pathology to its other senses. Sometimes a slight connotative shade causes preference for pathology or pathosis implying "some [as yet poorly analyzed] pathophysiologic process" rather than disease implying "a specific disease entity as defined by diagnostic criteria being already met". This is hard to quantify denotatively, but it explains why cognitive synonymy is not invariable.

Pathology (from the Ancient Greek roots of pathos, meaning "experience" or "suffering" and -logia, "study of") is concerned mainly with the causal study of disease.

A disease may be caused by external factors such as pathogens or by internal dysfunctions. 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 homeostatic processes. Commonly, the term is used to refer specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular organisms, and aberrant proteins known as prions. An infection or colonization that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal 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 diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease.

In biology, a pathogen (πάθος pathos "suffering, passion" and -γενής -genēs "producer of") is also known as an infectious agent, or a germ. In the oldest and broadest sense, a pathogen is anything that can produce disease; the term came into use in the 1880s.

Diseases can also be classified in other ways, such as communicable versus non-communicable diseases. 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 homeostatic processes. Commonly, the term is used to refer specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular organisms, and aberrant proteins known as prions. An infection or colonization that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal 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 diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease.

A non-communicable disease (NCD) is a disease that is not transmissible directly from one person to another.

The study of disease is called pathology, which includes the study of etiology, or cause.

In medicine, the term refers to the causes of diseases or pathologies.

A disease that cannot be cured. Incurable diseases are not necessarily terminal diseases, and sometimes a disease's symptoms can be treated sufficiently for the disease to have little or no impact on quality of life.

Terminal illness or end-stage disease is an incurable disease that cannot be adequately treated and is reasonably expected to result in the death of the patient.

A primary disease is a disease that is due to a root cause of illness, as opposed to secondary disease, which is a sequela, or 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 a bacterial infection, is causing a patient's secondary rhinitis when deciding whether or not to prescribe antibiotics.

Complication, in medicine, is an unfavorable evolution or consequence of a disease, a health condition or a therapy.

A primary disease is a disease that is due to a root cause of illness, as opposed to secondary disease, which is a sequela, or 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 a bacterial infection, is causing a patient's secondary rhinitis when deciding whether or not to prescribe antibiotics.

A sequela (, ; usually used in the plural, sequelae) is a pathological condition resulting from a disease, injury, therapy, or other trauma.

The terms Illness and sickness are both generally used as a synonym 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), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or 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 evolved responses—sickness behavior by the body—that helps clear infection and promote recovery. Such aspects of illness can include lethargy, depression, loss of appetite, sleepiness, hyperalgesia, and inability to concentrate.

Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease.

The terms Illness and sickness are both generally used as a synonym 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), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or 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 evolved responses—sickness behavior by the body—that helps clear infection and promote recovery. Such aspects of illness can include lethargy, depression, loss of appetite, sleepiness, hyperalgesia, and inability to concentrate.

Medicalization or medicalisation ([[American and British English spelling differences#-ise, -ize (-isation, -ization)|see spelling differences]]) is the process by which human conditions and problems come to be defined and treated as medical conditions, and thus become the subject of medical study, diagnosis, prevention, or treatment.

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 homeostatic processes. Commonly, the term is used to refer specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular organisms, and aberrant proteins known as prions. An infection or colonization that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal 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 diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease.

A passenger virus is a virus that is frequently found in samples from diseased tissue, such as tumours, but does not contribute to causing the disease.

The terms Illness and sickness are both generally used as a synonym 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), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or 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 evolved responses—sickness behavior by the body—that helps clear infection and promote recovery. Such aspects of illness can include lethargy, depression, loss of appetite, sleepiness, hyperalgesia, and inability to concentrate.

Sickness behavior is a coordinated set of adaptive behavioral changes that develop in ill individuals during the course of an infection.

A primary disease is a disease that is due to a root cause of illness, as opposed to secondary disease, which is a sequela, or 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 a bacterial infection, is causing a patient's secondary rhinitis when deciding whether or not to prescribe antibiotics.

Occasionally those with other health problems may develop pneumonia.

The terms Illness and sickness are both generally used as a synonym 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), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or 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 evolved responses—sickness behavior by the body—that helps clear infection and promote recovery. Such aspects of illness can include lethargy, depression, loss of appetite, sleepiness, hyperalgesia, and inability to concentrate.

Somnolence is often viewed as a symptom rather than a disorder by itself.

A medical condition is a broad term that includes all diseases, lesions, 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 disorders, uses the term general medical condition to refer to all diseases, illnesses, and injuries except for mental disorders. 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.

A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma.

A disorder is a functional abnormality or disturbance. Medical disorders can be categorized into mental disorders, physical disorders, genetic disorders, emotional and behavioral disorders, and functional disorders. 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 biological, social, and psychological factors in 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 disorders.

A functional disorder is a medical condition that impairs normal functioning of bodily processes that remains largely undetected under examination, dissection or even under a microscope.

The terms Illness and sickness are both generally used as a synonym 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), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or 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 evolved responses—sickness behavior by the body—that helps clear infection and promote recovery. Such aspects of illness can include lethargy, depression, loss of appetite, sleepiness, hyperalgesia, and inability to concentrate.

A subclinically infected person is thus an asymptomatic carrier of a microbe, intestinal parasite, or virus that usually is a pathogen causing illness, at least in some individuals.

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 homeostatic processes. Commonly, the term is used to refer specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular organisms, and aberrant proteins known as prions. An infection or colonization that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal 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 diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease.

However, in certain conditions, some species are thought to be capable of causing disease by causing infection or increasing cancer risk for the host.

Morbidity is a diseased state, disability, 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 ICU scoring systems. Comorbidity is the simultaneous presence of two or more medical conditions, such as schizophrenia and substance abuse.

In medicine, comorbidity is the presence of one or more additional diseases or disorders co-occurring with (that is, concomitant or concurrent with) a primary disease or disorder; in the countable sense of the term, a comorbidity (plural comorbidities) is each additional disorder or disease.

Pathosis (plural pathoses) is synonymous with disease. The word pathology also has this sense, in which it is commonly used by physicians in the medical literature, although some editors prefer to reserve pathology to its other senses. Sometimes a slight connotative shade causes preference for pathology or pathosis implying "some [as yet poorly analyzed] pathophysiologic process" rather than disease implying "a specific disease entity as defined by diagnostic criteria being already met". This is hard to quantify denotatively, but it explains why cognitive synonymy is not invariable.

Pathology is the medical discipline that describes conditions typically observed during a disease state, whereas physiology is the biological discipline that describes processes or mechanisms operating within an organism.

In epidemiology and actuarial science, the term "morbidity rate" can refer to either the incidence rate, or the prevalence of a disease or medical condition. 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.

It is distinct from "morbidity", which is either the prevalence or incidence of a disease, and also from the incidence rate (the number of newly appearing cases of the disease per unit of time).

The terms Illness and sickness are both generally used as a synonym 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), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or 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 evolved responses—sickness behavior by the body—that helps clear infection and promote recovery. Such aspects of illness can include lethargy, depression, loss of appetite, sleepiness, hyperalgesia, and inability to concentrate.

In medicine, a disease is considered asymptomatic if a patient is a carrier for a disease or infection but experiences no symptoms.