Hippocrates (yes, the guy who wrote the famous Hippocratic Oath) in the fifth century BC wrote of a case of what was very likely necrotizing fasciitis (NF). He didn’t use the modern name, of course, but this is NOT a new disease.
In the sense that the bacteria involved are commonly found in the environment, but it’s not well understood why most cases of, say, strep are just ordinary strep while a very few become NF. Or why most staph infections are ordinary staff but a few become NF. As noted by another poster, it’s not just one species of bacteria that causes this.
Also as previously noted, the bacteria aren’t “eating” the flesh, it’s actually toxins produced by the bacteria that kills body cells.
Washing/cleaning wounds is always a good idea, but combined with appropriate wound treatment a very important thing is to GO TO A DOCTOR if the wound/infection suddenly starts to spread, spreads rapidly, or is extremely painful all out of proportion to the size of the wound. Also if you start running a fever. The earlier you get treatment the more likely you are to survive, or survive intact.
In many instances it’s not just the bacteria that destroys limbs. You see, if you go into shock (which isn’t that unusual in these cases past a certain point) your blood pressure drops. As a result, your fingers and toes might die simply because your blood pressure can no longer push blood through the capillaries sufficiently well to keep the tissues supplied with oxygen. Or perhaps not well enough to even keep your internal organs going. When dropping blood pressure becomes life threatening they give you drugs to keep it high enough to keep you alive. The problem is, those same drugs severely restrict the capillaries in your extremities, which can, again, result in insufficient oxygen reaching your hands and feet. If this goes on long enough your fingers/toes and hands/feet simply die. Sometimes you have a choice between dying of shock, or killing off the ends of your limbs in order to survive. The doctors might also have to carve off large swaths of your torso if the NF moves to that part of your body, because any dead tissue has to be cut away to get the infection under control.
It really is a hideous thing.