One of the major problems that diabetics begin to face are problems with circulation. The overload of glucose in their blood stream begins to have a very deletorious effect on the various systems of capillaries in the body, whether they be in the legs causing circulation problems there and in their feet, or even in their eyes, one reason diabetics' sight begins to deteriorate, particularly if they are not successful in regulating their diet or taking control of their diabetes.
With appropriate diet and without the huge rushes of glucose into the blood, the problems can be avoided.
Diabetics frequently have a syndrome called diabetic neuropathy. This is when the nerves in all areas of the body are damaged due to high glucose concentrations (high blood sugar). Over time the sensory nerves lose the ability to function normally. The diabetic may experience numbness, tingling, and parasthesias. Because sensory nerves are damaged the feet of the diabetic (and other body parts) are unable to sense pain, temperature changes, and changes in pressure as acutely as someone who is not diabetic. For this reason, the feet sustain minor injuries that go unnoticed. When this occurs, the feet are more susceptible to infection beacause the initial injury went unnoticed. Once the foot becomes infected, diabetics have a harder time fighting the infection because of the high blood glucose concentrations.
To build upon what has already been said: Any problem with the circulation of the blood through the body decreases the body's ability to nourish its cells and to properly fight infection. The feet are the furthest extremity from the heart, and as such, blood flow is already less effective in the feet than in places nearer the body's core. The result is that in a diabetic's feet, the immune system can be so compromised that even a small infection (that a normal body would overcome) can become septic if untreated. Septic infections can be lethal in extreme circumstances, especially in people whose health is already compromised by other factors, like diabetes.