Hello, thanks for using health care magic From the description, it is more likely to be a fungal infection and particularly that of Candida. Candida infection affects particularly areas of skin fold with moisture and relatively poor hygiene. However, Candida or other fungal infections are sensitive to most antifungal drugs/creams including ketoconazole,miconazole, clotrimazole, terbinafide etc Also it could be possible its a chronic reaction to sweat is not uncommon particularly with people who do sweat a lot. It will be necessary to see a doctor for an evaluation or try -use of anti perspirants in the armpits -anti fungal drugs/creams; miconazole cream twice daily. If there is no improvement then see a doctor. Best regards and hope this helps you. Dr Achuo