Necrotizing fasciitis is caused by bacteria that enter the body through a break in the skin and rapidly destroy the soft tissue beneath it. In roughly 80% of cases, the infection starts at an identifiable wound site. The bacteria involved, the way they get in, and certain underlying health conditions all play a role in whether this rare but serious infection develops.
The Bacteria Behind the Infection
Necrotizing fasciitis falls into two broad categories based on which bacteria are responsible. Type I infections are polymicrobial, meaning multiple species of bacteria work together to cause tissue destruction. These infections typically involve a mix of aerobic and anaerobic bacteria, organisms that thrive with and without oxygen, respectively. Type I infections are more common in people with chronic health conditions like diabetes.
Type II infections are caused by a single bacterial species, most often Group A Streptococcus (the same bacterium behind strep throat). This form can strike otherwise healthy people and tends to progress extremely fast. Group A Strep is particularly dangerous because it produces a surface protein called M-protein that shields the bacteria from the immune system by blocking white blood cells from engulfing and destroying them. It also releases toxins known as superantigenic exotoxins, which trigger a massive, uncontrolled immune response. This flood of immune activity can cause rapid-onset shock and organ failure, and it’s a major reason why some cases deteriorate within hours.
How Bacteria Enter the Body
The bacteria that cause necrotizing fasciitis need a way in. According to the CDC, the most common entry points include:
- Burns
- Cuts and scrapes
- Insect bites
- Puncture wounds, including those from IV drug use
- Surgical wounds
The wound doesn’t have to be dramatic. Necrotizing fasciitis has been documented after minor procedures like a routine blood draw. It can also follow surgery or any invasive medical procedure where the skin barrier is compromised. In rarer cases, people develop necrotizing fasciitis after blunt trauma, an injury that doesn’t visibly break the skin at all. And in some cases, no injury can be identified. Most cases occur randomly, without a clear pattern or predictable trigger.
Seawater and Marine Bacteria
A distinct and particularly dangerous cause of necrotizing fasciitis involves Vibrio vulnificus, a bacterium that lives naturally in warm coastal waters. Vibrio infections spike between May and October, when water temperatures are higher. The bacterium is found in both salt water and brackish water (where rivers meet the ocean).
People typically get infected when an open wound comes into contact with contaminated coastal water. Vibrio vulnificus infections can escalate quickly. About 1 in 5 people with this type of infection die, sometimes within a day or two of getting sick. Many survivors require intensive care, and amputation is sometimes necessary to stop the spread of dead tissue. If you’ve been in coastal waters and develop signs of infection near a wound, telling your doctor about the water exposure is critical because Vibrio requires specific treatment.
Risk Factors That Increase Vulnerability
While necrotizing fasciitis can affect anyone, certain conditions make the body less able to fight off the initial bacterial invasion before it reaches deeper tissue. Diabetes is one of the most consistently identified risk factors. High blood sugar impairs immune cell function and damages small blood vessels, which reduces blood flow to the skin and soft tissue. This creates an environment where bacteria can multiply with less resistance.
Other conditions that raise risk include liver disease (especially cirrhosis), kidney disease, cancer, and any form of immunosuppression, whether from medications like corticosteroids and chemotherapy or from conditions like HIV. Peripheral vascular disease, which limits blood flow to the extremities, also makes tissue more susceptible. Chronic alcohol use increases risk both through liver damage and through the skin injuries and impaired wound healing that often accompany it. Obesity is another contributing factor, as it reduces blood flow to fatty tissue beneath the skin.
That said, Type II infections driven by Group A Streptococcus regularly occur in young, healthy individuals with no chronic conditions. Having no risk factors doesn’t eliminate the possibility.
How the Infection Destroys Tissue
What makes necrotizing fasciitis different from a typical skin infection is where and how fast the damage happens. The bacteria don’t stay at the skin surface. They migrate to the fascia, the thin layers of connective tissue that wrap around muscles, nerves, and blood vessels beneath the skin. Once established there, the bacteria release enzymes and toxins that break down tissue and destroy the small blood vessels that supply it.
As blood supply is cut off, the tissue dies. This creates a vicious cycle: dead tissue provides more nutrients for bacteria and removes the body’s primary way of delivering immune cells and antibiotics to the infection site. The destruction can spread along the fascial planes at a remarkable rate, sometimes several centimeters per hour. Because the damage begins deep, the overlying skin can initially look deceptively normal, which is one reason the infection is so often misdiagnosed early on.
The hallmark early symptom is pain that seems far more severe than what the visible wound would suggest. The skin may appear red and swollen at first, then progress to purple or dark discoloration as the underlying tissue loses blood supply. Blisters, skin that feels hard or wooden to the touch, and numbness in the affected area (as nerves are destroyed) are later signs that the infection has advanced significantly.
Why Speed of Treatment Matters
Because necrotizing fasciitis destroys tissue faster than almost any other soft tissue infection, treatment centers on emergency surgery to remove all dead and infected tissue. Antibiotics alone cannot reach the affected area once blood supply has been cut off. The combination of surgical removal and intravenous antibiotics is the standard approach, and the earlier surgery happens, the better the outcome. In cases involving Group A Streptococcus, doctors often add a specific antibiotic that can shut down the bacteria’s toxin production and reduce its ability to evade the immune system.
Mortality rates remain high even with aggressive treatment, particularly when diagnosis is delayed. The infection’s ability to mimic less serious conditions like cellulitis in its early stages is a major factor in late diagnosis. Doctors sometimes use a scoring system based on six blood test values, including white blood cell count, kidney function markers, blood sugar, and a marker of inflammation called C-reactive protein, to help distinguish necrotizing fasciitis from less dangerous infections when the clinical picture is unclear.