Illness (sometimes referred to as ill-health or ail) can be defined as a state of poor health.

It is sometimes considered a synonym for disease. Others maintain that fine distinctions exist. Some have described illness as the subjective perception by a patient of an objectively defined disease.

Introduction

The mode of being healthy includes, as defined by the World Health Organization, " 1 a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity". 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.

Physical

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.Hart, B. L. (1988) "Biological basis of the behavior of sick animals". Neurosci Biobehav Rev. 12: 123-137. PMID: 3050629 Johnson, R. (2002) "The concept of sickness behavior: a brief chronological account of four key discoveries". Veterinary Immunology and Immunopathology. 87: 443-450 PMID: 12072271 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.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