The role of pain in the survival of the individual was to send a message for the need to react accordingly following an assault or damage to the body. According to the definition of the International Association for the Study of Pain (IASP), pain “is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage”. This definition emphasizes, above all, the subjective nature of feeling pain. Each person reacts differently to a particular painful stimulus, based on past experiences and what is defined as their pain threshold, and each person is able to evaluate, according to their own parameter, how strong its pain the pain is, and so they are able to give it an objective measurement.

Pain can be classified according to whether it is acute or chronic in duration. Acute pain generally has a short duration and tends to resolve within days or weeks. Examples of acute pain include post-operative pain and post-traumatic pain. Chronic pain can be termed as a persistent discomfort from three to six months beyond the expected healing period. Chronic pain is the most common form of disability in the world and is often related to musculoskeletal disorders such as osteoarthritis. In addition, pain can be classified as mild, moderate, and severe depending on the intensity. Pain, in its many forms, is something a physician sees daily, and requires the patient to be treated appropriately for effectiveness and of quick action.

Harrison's Principles Of Internal Medicine.