Diabetes mellitus
Chronic complications are mostly caused by hyperglycemia (but not high enough to always cause acute complications). It causes damage to blood vessels and nerves. Damage to blood vessels can eventually cause strokes, heart attacks, kidney failure, blindness, slow healing of skin breaks - with the added possibility of infection - and even amputations from poor circulation (decreased blood flow, usually to the feet and toes). Damage to nerves can make diabetics not feel pain (this usually happens in their feet). This causes them to have more injuries and not realize they have hurt themselves. Damage to nerves can also cause pain even when there's no real injury. It's a type of phantom pain or ghost pain. The pain can be so intense that strong pain medication may be required.