Type 1 diabetes is an autoimmune disorder. It's believed that a combination of genetic predisposition and additional (as yet unidentified) factors provoke the immune system into attacking and killing the insulin-producing cells in the pancreas.
Type 2 diabetes is caused by a combination of insulin deficiency and insulin resistance (where the body can't use insulin as well as it should). As a result, glucose can't be moved from the blood into cells. Over time, the excess sugar in the blood gradually poisons the pancreas, causing it to make less insulin and making it even more difficult to keep blood glucose under control.
Obesity is a leading cause of insulin resistance. At least 80% of people with type 2 diabetes are overweight or obese. There are also genetic factors involved in the cause of type 2 diabetes. A family history of the disease has been shown to increase the chances of someone getting it.
People with type 1 diabetes who have not yet been diagnosed or who are not being treated (and therefore have high blood sugar levels) urinate frequently and feel excessively thirsty. They usually feel very tired and experience severe weight loss despite normal or excessive food intake.
The symptoms of type 2 diabetes usually appear more gradually. People with type 2 diabetes who do not have their blood glucose under control often have a persistent, mild thirst. They urinate frequently, and often feel mild fatigue and complain of blurred vision. Many women with the disease have recurring vaginal yeast infections.