Necrosis of tissue followed by infection by secondary spoilage bacteria and other factors resulting in specific morphological changes such as black and dark green is called gangrene. Necrotic tissue is decomposed by spoilage bacteria to produce hydrogen sulfide, which combines with iron decomposed in hemoglobin to form iron sulfide, giving the necrotic tissue a black color. Dry gangrene: Mostly seen at the ends of the extremities, such as in cases of atherosclerosis, thrombo-occlusive vasculitis and frostbite. At this time, the arteries are blocked while the venous return flow is smooth, so the necrotic tissue has less water, plus the body surface water is easy to evaporate, resulting in dry and crumpled lesions, dark brown, and there is a clear line of demarcation between the surrounding healthy tissue. Because the necrotic tissue is relatively dry, the infection by spoilage bacteria is generally light. Wet gangrene: Wet gangrene occurs in the internal organs (intestines, uterus, lungs, etc.) that are connected to the outside world and can also be seen in the extremities (when accompanied by bruising and edema). At this time, the necrotic tissue contains more water, so the infection by spoilage bacteria is severe, and the local swelling is obvious, dark green or dirty black. The decomposition of proteins by spoilage bacteria produces indole and fecal odor, causing bad odor. Due to the rapid development of lesions and diffuse inflammation, there is no obvious demarcation line between necrotic and healthy tissues. At the same time, tissue necrosis and decay of toxic products and bacterial toxins produced by the absorption, can cause systemic toxic symptoms, and even toxic shock and death can occur. Common wet gangrene are gangrenous appendicitis, intestinal gangrene, pulmonary gangrene and postpartum gangrenous endometritis. Gas gangrene: seen in gas gangrene bacilli invade traumatic wounds caused by rapid development and serious consequences, with or without gas production. The incubation period is 6 hours – 6 days, clinical symptoms are severe pain like swelling and cracking, the wound begins to red and swollen, the skin is pale and tense and shiny. Subsequently, the wound turns purple-black, blisters with dark red fluid appear, and foul-smelling fluid may flow. The muscles in the wound are dark red and swollen, losing elasticity, and the cut does not contract or bleed. At the later stage, systemic symptoms including toxemia appear. The treatment measures are mainly debridement and drainage, antibiotics, hyperbaric oxygen chamber, and amputation if necessary to enhance the patient’s chances of survival.