Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced. This high blood sugar produces the classical symptoms of polyuria (frequent urination), polydipsia (increased thirst) and polyphagia (increased hunger).

The three main types of diabetes mellitus (DM) are:

Other forms of diabetes mellitus include congenital diabetes, which is due to genetic defects of insulin secretion, cystic fibrosis-related diabetes, steroid diabetes induced by high doses of glucocorticoids, and several forms of monogenic diabetes.

All forms of diabetes have been treatable since insulin became available in 1921, and type 2 diabetes may be controlled with medications. Both types 1 and 2 are chronic conditions that usually cannot be cured. Pancreas transplants have been tried with limited success in type 1 DM; gastric bypass surgery has been successful in many with morbid obesity and type 2 DM. Gestational diabetes usually resolves after delivery. Diabetes without proper treatments can cause many complications. Acute complications include hypoglycemia, diabetic ketoacidosis, or nonketotic hyperosmolar coma. Serious long-term complications include cardiovascular disease, chronic renal failure, and diabetic retinopathy (retinal damage). Adequate treatment of diabetes is thus important, as well as blood pressure control and lifestyle factors such as smoking cessation and maintaining a healthy body weight.

Mellitus (died 24 April 624) was the first Bishop of London in the Saxon period, the third Archbishop of Canterbury, and a member of the Gregorian mission sent to England to convert the Anglo-Saxons from their native paganism to Christianity. He arrived in 601 AD with a group of clergymen sent to augment the mission, and was consecrated as Bishop of London in 604. Mellitus was the recipient of a famous letter from Pope Gregory I known as the Epistola ad Mellitum, preserved in a later work by the medieval chronicler Bede, which suggested the conversion of the Anglo-Saxons be undertaken gradually, integrating pagan rituals and customs. In 610, Mellitus returned to Italy to attend a council of bishops, and returned to England bearing papal letters to some of the missionaries.

Mellitus was exiled from London by the pagan successors to his patron, King Sæberht of Essex, following the latter's death around 616. King Æthelberht of Kent, Mellitus' other patron, died at about the same time, forcing him to take refuge in Gaul. Mellitus returned to England the following year, after Æthelberht's successor had been converted to Christianity, but he was unable to return to London, whose inhabitants remained pagan. Mellitus was appointed Archbishop of Canterbury in 619. During his tenure, he was alleged to have miraculously saved the cathedral, and much of the town of Canterbury, from a fire. After his death in 624, Mellitus was revered as a saint.