Iatrogenesis is the causation of a state of ill health or adverse effect or complication caused by or resulting from medical treatment. The word literally means "brought forth by a physician" (iatros means physician in Greek), though such conditions can also be the result of actions by other allied health sciences professionals, such as therapists, pharmacists, nurses, dentists. etc. From a sociological point of view there are three types of iatrogenesis, clinical iatrogenesis, social iatrogenesis, and cultural iatrogenesis.

Since Hippocrates's time, the potential nocive effect of a physician's actions has been recognized. The old mandate "do no harm" is an important clause of medical ethics, and iatrogenesis caused purposefully, or by avoidable error or negligence on the doctor's part became a punishable offence in many civilizations. Before medicine was a more exact and scientific subject, however, most medical actions were outright nocive to the patient, such as the widespread practices of bloodletting, purging, branding of wounds with fire, and many others. Well until the discovery of anesthesia and asepsis, going to hospitals was a truly dangerous thing, due to the trauma of surgery and the ever present risk of nosocomial infection.

With the development of scientific medicine in the 20th century, it could be expected that iatrogenesis would be more easily avoided. With the discovery of asepsis, anesthesia, antibiotics, new and better surgical techniques, etc., this really happened to a great extent, and mortality and morbidity brought forth by the physician acts decreased enormously. Unfortunately, though, due to the exponential increase in complexity and in the number of drugs, medical procedures, etc., iatrogenesis became an unavoidable part of medical care, no longer with one or two, but with many possible sources of damage to the patient's health and well-being.

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