The bacteria can enter any open wound - a pimple, and ingrown hair, a hang nail. It only takes one entity, and it multiplies like crazy.
My sister-in-law is an RN. She's seen it. It spreads through hospitals and doctors offices. People sit in the chairs wearing shorts. If they cut their leg saving - they get it. If a man has an ingrown hair, he's got it, too. It spreads that easily. Just rubbing against someone could pass it on.
She says the infection has to be drained and packed for weeks - that's if you catch it early. Otherwise, the dead flesh has to be cut out. She says it's pretty gross stuff.
The husband of a woman who works with my daughter has had his foot cut off. They couldn't stop the infection. Last I knew, he was still in the hospital. Maybe they didn't get it all.