Type 1 diabetes, also known as juvenile diabetes, as it is prevalent among young people, is an insulin dependent disease suffered by only 10% of the diabetic population. Research is still ongoing into the cause of type 1 diabetes, but it is thought to be a virus which triggers the immune system into killing not only the virus, but also the insulin-producing cells of the pancreas (beta cells) in the Islets of Langerhans. This means the body cannot produce insulin to help the glucose in the blood diffuse into the cells. The cells need the glucose to turn it into energy for the body. This is the reason that one of the symptoms of type 1 diabetes is tiredness.
The picture below shows a cross-section of an Islet in a healthy pancreas on the left compared with an Islet of a pancreas, which has been infiltrated by immune cells (T-cells and macrophages) on the right. The immune cells are the dark stained cells.
Eventually, the immune cells will destroy almost all of the beta cells until insulin production is so low that glucose can't be diffused into the cells. At this stage, the patient can be described as a type 1 diabetic.