Diabetes mellitus is a metabolic disorder characterized by hyperglycemia (high glucose blood sugar), among other signs. The World Health Organization recognizes three main forms of diabetes: type 1, type 2 and gestational diabetes (or type 3, occurring during pregnancy), although these share signs and symptoms but have different causes and population distributions. They are not a single disease or condition. Type 1 is generally due to autoimmune destruction of the insulin-producing cells - pancreatic beta cells - while type 2 is characterized by tissue wide insulin resistance and varies widely. Gestational diabetes is due to a poorly understood interaction between fetal needs and maternal metabolic controls. Type 2 sometimes progresses to loss of beta cell function as well.