“Flesh eating bacteria” or NF isn’t directly contagious. More to the point, it’s more than one bacteria that requires a specific combination in a specific host to take off.
The research I’ve seen on this says that what determines whether NF explodes in a host is the host. Some hosts are “sweeter” than others, or something near to that effect.
That’s why you don’t see NF in clusters. It’s not moving from host to host. Rather, it’s a combination of bacteria common to humans that in certain humans catches fire in the right combination.
One wonders if changing the body's pH in a hurry would therefore halt it.