These words all refer to poor health or to a particular episode of bad health. Sickness and illness are the most informal of these terms; both refer to an episode of bad health, no matter what its duration. While the words are used interchangeably, sickness might be the more usual and general choice, illness the slightly more formal one. Also, since sickness can sometimes refer specifically to nausea alone, illness is sometimes used to avoid this implication when it would be inappropriate: overcome with sickness shortly after eating the contaminated food; an illness that can result in total blindness if left untreated. Furthermore, sickness can sometimes imply an episode that temporarily makes one unable to function, whereas illness can imply a longer-lasting siege that is accompanied by impairment but not cessation of normal functioning: a sickness that kept him in bed for two weeks; an illness that caused him little difficulty in his youth but began to take its toll as he reached middle age. Sickness can suggest external causation of the acute episode, while illness can suggest inherent weakness or malfunctioning as the source of chronic poor health. Also, illness is the word of choice for all mental disturbance, form mild neurosis to severe psychosis: the growing incidence of mental illness . When sickness is substituted for this neutral use of illness , an emotional colouration is added, implying greater seriousness or urgency or suggesting an attitude of condemnation: These monstrous crimes should be testimony enough to the killer’s sickness . With a similar emphasis, sickness is often used in a more general way: a pervasive sickness in society that predisposes people to violence as a cure-all for their frustrations and discontents.

Disease is often popularly thought to apply only to sickness that is infectious or communicable: disease bred by poor sanitation and improper sewage disposal. But disease can refer as widely as sickness or illness to any kind of bad health, with the advantage that its very generality yield no implications as to whether the sickness is acute or chronic, mild or harsh, or long or short in duration: a form of heart disease caused by a genetic defect; a case of Parkinson’s disease ; cancer and other diseases whose ultimate causes are still unknown; such mild viral diseases as the common cold. By contrast, disorder usually refers to a malfunction of mind or body that may be mild or serious, infectious or inherent, but is seen in some imbalance, as a metabolic or chemical defect, or in the improper working of some mechanism: hormonal disorders such as cretinism; a mental disorder typified by delusions of grandeur and aural hallucinations. The word can be useful because it leaves the question of cause open and points strictly to symptoms indicating that something is awry.

Malady is a more formal synonym for disease that may seem outdated in descriptive use, although it has an emotional note of alarm that makes it useful in metaphorical situations: the denial of female sexuality that is a malady endemic to Western civilization. Malaise refers to an indefinable sense of ill-being: a predictable malaise that is the first sign of the onset of the disease. It aptly describes a psychological state in which someone feels ill at ease or disquieted for whatever reason: an abiding malaise that jaundiced his whole view of world affairs.

Infirmity applies most concretely to a weakness of mind or body, but nowadays it may sound too genteel as a substitute for disease or disorder ; this is true, as well, in its extended uses: an infirmity that kept him in a wheelchair for a number of years; needless cruelty that mocks at the infirmities of others; his tendency to exaggerate ?the one infirmity in an otherwise admirable personality. Ailment can refer to a symptom or collection of symptoms that causes noticeable discomfort to someone: What exactly is your ailment ? This word, too, can sound outdated, although sometimes it can have an informal or regional thrust: His lumbago is the one ailment that keeps him on edge day in, day out. The word formerly focused more clearly on the enervation or depletion of one’s sense of well-being. Complaint now functions as an informal substitute for symptom: Frequent faintness was a complaint she learned to live with. Sometimes, the word can specifically indicate symptoms that are confided to one’s doctor: cards on which he carefully noted every complaint of his patients.