Underarm acne can be caused by diseases such as folliculitis, boils, and sweatshirts. 1. Folliculitis: Folliculitis is an inflammatory skin disease mainly involving hair follicles, usually caused by pathogen infection. It manifests as red papules centered on hair follicles, accompanied by itching and pain, and is most common on the head, face, and armpits. 2. Boil: Boil is an acute purulent inflammation of the hair follicle and its deep surrounding tissues, usually caused by Staphylococcus aureus infection, the patient will appear slightly elevated red painful nodules, occurring in any part of the head, face, neck, and some patients will appear in the armpit. 3. Sweatshirt: Sweatshirt mostly occurs in the bilateral armpits, the etiology is complex, may be related to hormonal imbalance in the body, embryonic dysplasia, local moisture, etc., with erythema edematous papules and plaques or recurrent painful, erythema nodosum main symptom. There are more causes of underarm acne (such as prickly heat, etc.), and it is recommended to seek medical examination for a clear diagnosis.