I don't know where it came from but I suffered from constantly reocurring bouts of it in the corner of my mouth for at least a couple of years. Finally went to the doctor who prescribed an anti-fungal cream (He had correctly id'd it as Angular Cheilitis but the cream did Nothing.) I suffered further until I came upon an unrelated article about an oncologist in Rome who believes that many types of cancer are fungal. (google it) For years he's treated and cured early stage cases with a product you likely have in your kitchen.
So I tried it on the sore on my mouth. The product is Sodium Bicarbonate - baking soda. That's it!
I rubbed some on with my finger and left the slightest bit in the sore for a while. It was mostly healed the next day. It hasn't reocurred. That's all I did and a complete cure was the result. It hasn't come back. So whatever stage of sores you have, you have nothing to lose by trying this first. Suffer no more. Smile again easily!