Diabetes mellitus is a disease characterized by persistent hyperglycemia (high blood sugar levels). It is a metabolic disease that requires medical diagnosis, treatment and lifestyle changes. The World Health Organization recognizes three main forms of diabetes: type 1, type 2 and gestational diabetes (or type 3, occurring during pregnancy), although these three "types" of diabetes are more accurately considered patterns of pancreatic failure rather than single diseases. Type 1 is due to autoimmune destruction of the insulin-producing cells, while type 2 and gestational diabetes are due to insulin resistance by tissues.

Since the first therapeutic use of insulin (1921) diabetes has been a treatable but chronic condition, and the main risks to health are its characteristic long-term complications. These include cardiovascular disease (doubled risk), chronic renal failure (it is the main cause for dialysis in developed world adults), retinal damage which can lead to blindness and is the most significant cause of adult blindness in the non-elderly in the developed world, nerve damage, erectile dysfunction (impotence), to gangrene with risk of amputation of toes, feet, and even legs.

Terminology
The term 'diabetes' was coined by Aretaeus (81?133 CE) of Cappadocia. The Greek word diaba?nein literally means "passing through," or "siphon," a reference to one of diabetes' major symptoms?excessive urine production. The word became "diabetes" from the English adoption of the medieval Latin diabetes. In 1675 Thomas Willis added mellitus from the Latin word for honey (mel in the sense of "honey sweet") when he noted that the blood and urine of a diabetic has a sweet taste. This had been noticed long before in ancient times by the Greeks, Chinese, Egyptians, and Indians. In 1776 it was confirmed the sweet taste was because of an excess of a kind of sugar in the urine and blood of people with diabetes.

The ancient Indians tested for diabetes by observing whether ants were attracted to a person's urine, and called the ailment "sweet urine disease" (Madhumeha); medieval European doctors tested for it by tasting the urine themselves, a scene which was occasionally depicted in Gothic reliefs.

While the term diabetes without a modifier usually refers to diabetes mellitus, there is another, rarer condition named diabetes insipidus (unquenchable diabetes) in which the urine is not sweet; it can be caused by either kidney (nephrogenic DI) or pituitary gland (central DI) damage.

History
Although diabetes has been recognized since antiquity, and treatments of various efficacy have been known since the Middle Ages, the elucidation of the pathogenesis of diabetes occurred mainly in the 20th century.

The discovery of the role of the pancreas in diabetes is generally ascribed to Joseph von Mering and Oskar Minkowski, European researchers who in 1889 found that when they completely removed the pancreas of dogs, the dogs developed all the signs and symptoms of diabetes and died shortly afterward. In 1910, Sir Edward Albert Sharpey-Schafer of Edinburgh suggested that people with diabetes were deficient in a single chemical that was normally produced by the pancreas'he proposed calling this substance insulin. The term is derived from the Latin insula, meaning island, in reference to the islets of Langerhans in the pancreas that produce insulin.
The endocrine role of the pancreas in metabolism, and indeed the existence of insulin, was not fully clarified until 1921, when Sir Frederick Grant Banting and Charles Herbert Best repeated the work of Von Mering and Minkowski, but went a step further and demonstrated that they could reverse induced diabetes in dogs by giving them an extract from the pancreatic islets of Langerhans of healthy dogs. Banting, Best, and colleagues (particularly Collip) went on to isolate the hormone insulin from bovine pancreases at the University of Toronto in Canada. This led to the availability of an effective treatment?insulin injections?and the first clinical patient was treated in 1922. For this, Banting et al received the Nobel Prize in Physiology or Medicine in 1923; both shared their Prize money with others in the team who were not recognized. Banting and Best made the patent available without charge and did not attempt to control commercial production. Insulin production and therapy rapidly spread around the world, largely as a result of this decision.

Despite the availability of treatment, diabetes remained a major cause of death. For instance, statistics reveal that the cause-specific mortality rate during 1927 amounted to about 47.7 per 100,000 population in Malta.

The distinction between what is now known as type 1 diabetes and type 2 diabetes was made by Sir Harold Percival (Harry) Himsworth in 1935 and the findings were published in January 1936.