That "stomach bug" wasn't a stomach bug at all. My dad had been dealing with pain in his groin, and he'd lied about it because he was embarrassed. By the time it got bad enough that my mom forced him to show her what was wrong, his penis had retracted entirely into his body, and his testicles had swollen to the size of a grapefruit and turned black. The parts that weren't black were an angry, swollen red.
Even at the hospital, the doctors didn't know what was wrong. He went in late that evening and almost immediately had a team of doctors around him. Waiting for five hours at the ER isn't fun, but it turns out that getting treated immediately is a really bad sign. His heart started beating wildly, his kidneys started fucking up, and before long he was on oxygen -- then a ventilator. By the time they resorted to having a machine do the lunging for him, we knew he was dying. We didn't know why, though, and for a while, it didn't look like he'd live. The hospital assigned a nurse to my mom to keep her calm.
This proved impossible, even with the ER's medicinal kittens.
Untreated, necrotizing fasciitis has a 100 percent mortality rate. With modern medicine on their side, "only" 30-40 percent of sufferers die. My dad's case happened because his (undiagnosed) diabetes lowered his immune system's defenses enough to let the streptococcus in. About half of the people who come down with Fournier's Gangrene (the fancy term for "literal crotch rot") have diabetes.
But even if you're the spitting image of health, Mr. Strepto is just waiting to make a ruin out of your coccus. Between 15-30 percent of the population carry flesh-eating bacteria around in their daily lives without suffering any symptoms. Then their immune system gets compromised, or they wind up with an open wound, and BAM. One terrifying case you can read about involved a healthy 29-year-old who made the fool choice to masturbate with soap instead of lube. Go ahead, click that link. I dare you.