While the causes of type 1 diabetes are unknown, doctors do know how the disease affects the body and that it is an autoimmune disorder. In this case, high blood sugar is caused when the immune system sends antibodies to destroy the insulin-producing cells in the pancreas. The lack of insulin makes it impossible for the cells to absorb glucose (energy) from the blood stream.
The glucose that was not used in the cells builds up in the blood, causing serious side effects. Doctors speculate that the causes of type 1 diabetes include genetics and perhaps viruses.