Gas (“wet”) gangrene is the most life-threatening form of gangrene. It occurs in wounds that are infected by a family of bacteria called Clostridium. At least 20 kinds of Clostridium can cause gas gangrene. What makes the clostridia special is that they are anaerobic – that is, they grow best in the absence of oxygen. Because healthy human tissue is full of oxygen, it's rarely bothered by these bacteria. However, when they do grow, clostridia release gas and poisons into the body. This form of gangrene spreads very quickly, and can cause a rapid death.
Injuries that leave a portion of muscle tissue deprived of oxygen make an ideal home for clostridia. Some of the cells in a wound are already dead, and the infection starts in these. "Crush" injuries are most likely to cut off blood supply to a large amount of tissue, and are most prone to gangrene. The infection usually strikes the extremities, where blood supply is most easily cut off. Car accidents, for instance, often cause crush injuries to the lower legs - exactly the sort of wound that clostridia find most suitable.
Surgery can also produce areas of vulnerable tissue. Conditions favourable to clostridia are most common in people who have preexisting vascular problems, or who have many of the risk factors for heart disease. For example, high cholesterol can make blood more likely to clot. A person with high cholesterol is more likely to have more clotted blood vessels after surgery. The tissue that's normally fed by those blood vessels can run out of oxygen, allowing bacteria to multiply.
There are other causes of “wet” gangrene that aren't associated specifically with clostridia. Other bacteria such as E.coli, Klebsiella, and Proteus may cause infected “wet” gangrene in damaged tissue, resulting in a serious, often life-threatening infection.
In contrast, “dry” gangrene occurs when the blood supply to tissue is cut off but no infection develops. In these cases the tissue dies but the person generally feels well overall. Serious diseases that interfere with blood flow can cut off oxygen supply to the extremities, even when there's no trauma or injury. For example, peripheral vascular disease in which blood flow to the legs is poor can deprive a foot or leg of blood or oxygen, resulting in gangrene. People with diabetes, for instance, are prone to gangrene in the foot. Diabetes-related gangrene is the most common reason for foot amputation. Severe frostbite (due to overexposure to the cold) can also lead to gangrene due to lack of proper blood flow to the tissues.