Type 1 Diabetes is a chronic disease that occurs either when the pancreas does not produce enough insulin or when the body cannot effectively use the insulin it produces. Insulin is a hormone required for the body to regulate and use blood sugar.
The classic symptoms are frequent urination, increased thirst, constant hunger, weight loss, vision changes and fatigue. It's a major cause of blindness, kidney failure, heart attacks, stroke and frequently leads to lower limb amputation.
It's believed to affect around 8% of the global population (more than 400 million) and the incidence is increasing.
Neither the cause of Type 1 diabetes nor the means to prevent it are known. Source: World Health Organization
A number of possible theories of its cause(s) have been suggested, including genetic susceptibility, environmental factors, and exposure to an antigen.
Further info. at Wikipedia