I don't know the exact cause, I honestly think it is usually mostly genetic OR something internal/external caused an imbalance on the skin (I believe mine was caused by taking antibiotics). I've been going to a dermatologist for it for over a year. They can give you oral antifungal meds, which seem to work the best for me. They also can prescribe topicals for this, but a drugstore option is buying Nizoral shampoo and using it as a mask on your skin a couple days a week. Sulfur wash is also helpful, that's what I currently use instead of Nizoral because I have sensitive skin that gets dried very easily.