Diabetes is a disease that affects the way your body makes and uses insulin. Either your body doesn’t make enough insulin or it doesn’t use it properly, or both. This causes excess sugar to build up in your blood. When untreated, blood sugar levels can rise to abnormal levels and can cause complications. Complications can include: stroke, eye disease, heart disease, high blood pressure, kidney disease, reduced blood flow to the legs (causing pain or limb loss), and peripheral nerve disease.