Diabetes is a disorder of carbohydrate metabolism characterised by impaired ability of the body to produce or respond to insulin and thereby maintain proper levels of sugar (glucose) in the blood.

There are two major forms of the disease. Type I diabetes, formerly referred to as insulin-dependent diabetes mellitus (IDDM) and juvenile-onset diabetes, which usually arises in childhood. It is an autoimmune disorder in which the diabetic person's immune system produces antibodies that destroy the insulin-producing beta cells. Because the body is no longer able to produce insulin, daily injections of the hormone are required.

Type II diabetes, formerly called non-insulin-dependent diabetes mellitus (NIDDM) and adult-onset diabetes, usually occurs after 40 years of age and becomes more common with increasing age. It arises from either sluggish pancreatic secretion of insulin or reduced responsiveness in target cells of the body to secreted insulin. It is linked to genetics and obesity, notably upper-body obesity. People with type II diabetes can control blood glucose levels through diet and exercise and, if necessary, by taking insulin injections or oral medications.