Hello and welcome to Healthcare Magic . You seem to be suffering from Taenia axillaris .It is a fungal infection (ringworm variety ) common in young adults in skin folds . In terms of an armpit fungus, local conditions (like tight clothing , sweat,an allergy to a cosmetic etc )are the main contributing factors which may initially cause an armpit rash that is then infected by a fungus. Often, an armpit fungus may occur as a result of secondary spread, where fungal strands or spores are transferred from other areas of the body due to scratching with the fingers or using a towel on both areas after bathing. Sometimes underlying disorders like Anemia ,Diabetes , stress etc can cause frequent and slow healing skin eruptions .So it is advisable to go for complete blood counts and rule out Hypothyroid and Diabetes . Hope this helps .