A furuncle is a deep, painful skin infection affecting a hair follicle and the surrounding area. A more severe form, in which several hair follicles are affected, is called a carbuncle. The causative organism is often a bacterium called S. aureus. It's most commonly seen on the neck, back and buttocks. Commonly known as a boil, this condition can lead to more serious health problems if left untreated.
According to legend, Ottoman Sultan Selim I died following a boil that developed on his back. As the story goes, he had a pimple on his back squeezed, and the infection deepened and progressed. (It's worth remembering that antibiotics didn't exist at the time.)