Cause
Risk factors
More than 70% of cases are recorded in people with at least one of these clinical situations: immunosuppression, diabetes, alcoholism/drug abuse/smoking, malignancies, and chronic systemic diseases. For reasons that are unclear, it occasionally occurs in people with an apparently normal general condition.Necrotizing fasciitis can occur at any part of the body, but it is more commonly seen at the extremities, perineum, and genitals. Only a few of such cases arise from the chest and abdomen. Trauma is the usual cause of the infection, such as from intravenous drug injection, insulin injection, animal and insect bites, catheter insertion over the skin, or a fistula connecting skin to the internal body organs. Skin infections such as abscess and ulcers can also complicate necrotizing fasciitis. Spreading of infection through blood has been suggested for those with streptococcal pharyngitis. For infection of the perineum and genitals (Fournier gangrene), trauma, surgery, urinary tract infection, stones, and Bartholin gland abscess are the usual causes.The risk of developing necrotizing fasciitis from a wound can be reduced by good wound care and handwashing. Bacteria
Types of soft-tissue necrotizing infection can be divided into four classes according to the types of bacteria infecting the soft tissue. This classification system was first described by Giuliano and  his colleagues in 1977.Type I infection: This is the most common type of infection, and accounts for 70 to 80% of cases. It is caused by a mixture of bacterial types, usually in abdominal or groin areas.
Myonecrotic infections caused by these clostridial species commonly occur in injecting heroin users. Those with clostridial infections typically have severe pain at the wound site, where the wound typically drains foul-smelling blood mixed with serum (serosanguinous discharge). Shock can progress rapidly after initial injury or infection, and once the state of shock is established, the chance of dying exceeds 50%. Another bacterium associated with similar rapid disease progression is group A streptococcal infection (mostly Streptococcus pyogenes). Meanwhile, other bacterial infections require two or more days to become symptomatic.Type II infection: This infection accounts for 20 to 30% of cases, mainly involving the extremities. This mainly involves  Streptococcus pyogenes bacteria, alone or in combination with staphylococcal infections. Both types of bacteria can progress rapidly and manifest as toxic shock syndrome. Streptococcus species produce M protein, which acts as a superantigen, stimulating a massive systemic immune response which is not effective against the bacterial antigen, precipitating shock. Type II infection more commonly affects young, healthy adults with a history of injury.Type III infection: Vibrio vulnificus, a bacterium found in saltwater, is a rare cause of this infection, which occurs through a break in the skin. Disease progression is similar to type II but sometimes with little visible skin changes.Type IV infection: Some authors have described the type IV infection as fungal in nature. Diagnosis
Early diagnosis is difficult, as the disease often looks early on like a simple superficial skin infection.