I have this under my armpits.
Had it for years. As with acne never really worked out what causes it apart from the fact that i sweat alot. But sweatin a lot isnt enough in itself to actually cause it.
My answer has been to shave my arm pits for years but now and hten i can't be bothered and eventually it will usually come back.
Always wondered if it comes internally or not even though it isn't supposed to. Also this would mean the bacteria would be in my blood which i'm not sure of possiblke or not.
Maybe it is connected to the acne - Maybe something to do with bacterial overgrowth.
Something else i need to spend more time with.
Most liekly nothing to do with acne though.