Type I diabetes is an autoimmune disease; the body’s immune cells attack the β cells in the Isles of Langerhans in the pancreas. The body soon can no longer produce insulin, and eventually the patient, if untreated, dies either from starvation or from diabetic ketoacidosis. (A certain minimum level of insulin in the body is required for health.)
Type II diabetes is a disease of carbohydrate intolerance. A carbohydrate is a string of glucose molecules, and excessive glucose in the blood is dangerous, potentially fatal. So the body secretes insulin to move the glucose into the muscle to be metabolized, or else into the fat tissue to be stored as triglycerides. When the muscles fill up with glucose or glycogen (the storage form of glucose), they become insulin-resistant, and it takes more and more insulin to force them to accept more glucose. The liver cannot accept glucose past a certain point and it, too becomes insulin-resistant (the liver also becomes insulin-resistant if required to deal with too much fructose—table sugar is half-glucose and half-fructose). The fat cells eventually reach their capacity to store fat, and then they, too, become insulin resistant. And so it goes.
Insulin resistance begins years before the blood glucose rises enough for a diagnosis of Type II, and the high level of insulin takes its toll on all the body’s systems. Furthermore, the strain of producing enough insulin to force the muscles, organs, and fat tissue to accept more glucose damages the pancreas, and the β cells start to die. If untreated, Type II diabetes can eventually kill them all off, though fortunately for Type II diabetics, this process takes quite some time. By the time Type II has advanced to this point, the constant high insulin level in the body has called all sorts of other damage, as well. Part of the problem is that Type II is really a problem of excessive insulin, but the medical profession operates on the assumption that it is a problem of excessive glucose, and the resulting treatment with additional glucose really only worsens the damage.