One third of the 21 million people with diabetes don't know they have it. Uncontrolled diabetes damages blood vessels and other parts of your body. Even if you have been diagnosed with diabetes and feel OK, you still need to take extremely good care of your disease. The damage from diabetes may take years to show, and you may not be able to undo such conditions as heart disease and nerve damage. Only a doctor can tell you for sure if you have diabetes.