A furuncle is a skin infection of a single hair follicle, usually caused by Staphylococcus aureus. It presents as a red, painful nodule with a central punctum that can become fluctuant with pus. Risk factors include diabetes, HIV/AIDS, steroid use, and other immunosuppressive conditions. Treatment involves incision and drainage to remove pus, followed by antibiotics such as flucloxacillin or erythromycin. Without treatment, furuncles may burst and drain spontaneously, healing slowly with scarring.