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Illness (sometimes referred to as ill-health or ailment) is a state of poor health. Illness is sometimes considered a synonym for disease.[1] Others maintain that fine distinctions exist.[2] Some have described illness as the subjective perception by a patient of an objectively defined disease.[3]


The mode of being healthy includes, as defined by the World Health Organization, " [...] a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity". [4] When these conditions are not fulfilled, then one can be considered to have an illness or be ill. Medication and the science of pharmacology is used to cure or reduce symptoms of an illness or medical conditions. Developmental disability is a term used to describe severe, life-long disabilities attributable to mental and/or physical impairments.


Conditions of the body or mind that cause pain, dysfunction, or distress to the person afflicted or those in contact with the person can be deemed an illness. Sometimes the term is used broadly to include injuries, disabilities, syndromes, infections, symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts these may be considered distinguishable categories. A pathogen or infectious agent is a biological agent that causes disease or illness to its host. A passenger virus is a virus that simply hitchhikes in the body of a person or infects the body without causing symptoms, illness or disease. Foodborne illness or food poisoning is any illness resulting from the consumption of food contaminated with pathogenic bacteria, toxins, viruses, prions or parasites.

Adaptive response

According to evolutionary medicine, much illness is not directly caused by an infection or body dysfunction but is instead a response created by the body. Fever, for example, is not caused directly by bacteria or viruses but the body (after having immunologically identified their presence) seeking to clear itself of them through raised body temperature. Evolutionary medicine identifies a set of responses that aids fever in doing this called sickness behavior.[5][6][7] These include such illness defining health changes as lethargy, depression, anorexia, sleepiness, hyperalgesia, and the inability to concentrate. These together with fever are caused by the brain through its top down control upon the body. They are, therefore, not necessary, and often do not accompany an infection (such as the lack of fever during malnutrition or late pregnancy) when they have a cost that outweighs their benefit. In humans, an important factor are beliefs that influence whether the health management system in the brain that evaluates costs and benefits deploys them or not. The health management system, when it factors in false information, has been suggested to underlie the placebo reduction of illness.[8]


Mental illness (or Emotional disability, Cognitive dysfunction) is a broad generic label for a category of illnesses that may include affective or emotional instability, behavioral dysregulation, and/or cognitive dysfunction or impairment. Specific illnesses known as mental illnesses include major depression, generalized anxiety disorder, 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 impact one’s ability to work or go to school and contribute to problems in relationships. Other generic names for mental illness include “mental disorder”, “psychiatric disorder”, “psychological disorder”,“abnormal psychology”, “emotional disability”, “emotional problems”, or “behavior problem”. The term insanity is used technically as a legal term. Brain damage may result in impairment of mental function.


Social determinants of health are the social conditions in which people live which determine their health. Illnesses are generally related to social, economic, political, and 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.


Health care is the prevention, treatment, and management of illness and the preservation of mental and physical well-being through the services offered by the medical, nursing, and allied health professions. The organised provision of such services may constitute a health care system. Before the term "healthcare" became popular, English-speakers referred to medicine or to the health sector and spoke of the treatment and prevention of illness and disease. A patient is any person who receives medical attention, care, or treatment. The person is most often ill or injured and is being treated by, or in need of treatment by, a physician or other medical professional. Health consumer or health care consumer is another name for patient, usually used by some governmental agencies, insurance companies, and/or patient groups.

Medical emergencies are injuries or illnesses that pose an immediate threat to a person's health or life which require help from a doctor or hospital. The doctor's specialization of emergency medicine includes techniques for effective handling of medical emergencies and resuscitation of patients. Emergency departments provides initial treatment to patients with a broad spectrum of illnesses and injuries, some of which may be life-threatening and requiring immediate attention.

A drug is any chemical substance other than a food or device that affects the function of living things. Drugs can be used to treat illness, or they can be used recreationally to alter behavior and perception. Medications are typically produced by pharmaceutical companies and are often patented. Those that are not patented are called generic drugs. Some drugs, if misused, can overwhelm the homeostasis of a living organism, causing severe illness or death. Essentially it is a type of poisoning. In the context of biology, poisons are substances that can cause illness.

Bedrest as a medical treatment refers to staying in bed day and night. Even though most patients in hospitals spend most of their time in the hospital beds, bedrest more often refers to an extended period of recumbence at home.

Human enhancement technologies (HET) are technologies that can be used not simply for treating illness and disability, but also for enhancing human capacities and characteristics. Medication is a licenced drug taken to cure or reduce symptoms of an illness or medical condition. A wheelchair is mobility device that takes the form of a chair on wheels, used by people for whom walking is difficult or impossible due to illness or disability.

Shock therapy is the deliberate and controlled induction of some form of physiological state of shock in an individual for the purpose of psychiatric treatment. Electrotherapy is the use of electrical energy in the treatment of impairments of health and a conditions of abnormal functioning.

Study of illness

Epidemiology is the scientific study of factors affecting the health and illness of individuals and populations, and serves as the foundation and logic of interventions made in the interest of public health and preventive medicine.

Behavioral medicine is an interdisciplinary field of medicine concerned with the development and integration of psychosocial, behavioral and biomedical knowledge relevant to health and illness. Clinical Global Impression scale to assess treatment response in patients with mental disorders. It's " Improvement scale" requires the clinician to rate how much the patient's illness has improved or worsened relative to a baseline state. Mental confusion and decreased alertness may indicate that a chronic illness has gotten worse.

Religion and illness

Jewish and Islamic law grant exceptions to people of ill health. For example, one whose life would be endangered by doing so is exempt from (and indeed forbidden to participate in) fasting on Yom Kippur or during Ramadan.

In the New Testament Jesus is described as performing miracles of healing.

Illness was one of the four scenes, referred to as the four sights, encountered by Gautama Buddha.

Korean Shamanism involves notions of "spirit sickness".

Folk medicine is collectively procedures traditionally used for treatment of illness and injury, aid to childbirth, and maintenance of wellness. It is a body of knowledge distinct from "scientific medicine" and may coexist in the same culture.

The border between normality and illness may be subjective. For example, in some religions, homosexuality is believed to be an illness.[citation needed]

See also


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  1. illness at Dorland's Medical Dictionary
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  4. WHO, 1946
  5. Hart, B. L. (1988) "Biological basis of the behavior of sick animals". Neurosci Biobehav Rev. 12: 123-137. PubMed
  6. Johnson, R. (2002) "The concept of sickness behavior: a brief chronological account of four key discoveries". Veterinary Immunology and Immunopathology. 87: 443-450 PubMed
  7. Kelley, K. W., Bluthe, R. M., Dantzer, R., Zhou, J. H., Shen, W. H., Johnson, R. W. Broussard, S. R. (2003) "Cytokine-induced sickness behavior". Brain Behav Immun. 17 Suppl 1: S112-118 PubMed
  8. Humphrey, Nicholas. (2002) "Great Expectations: The Evolutionary Psychology of Faith-Healing and the Placebo Effect", in The Mind Made Flesh: Essays from the Frontiers of Psychology and Evolution, chapter 19, pages 255-85, Oxford University Press ISBN 978-0192802279