called an abscess) under the skin. Usually single, a carbuncle is most likely to occur on a hairy area of the body such as the back or nape of the neck. But a carbuncle also can develop in other areas of the body such as the buttocks, thighs, groin, and armpits.
Most carbuncles are caused by Staphylococcus aureus bacteria, which inhabit the skin surface, throat, and nasal passages. These bacteria can cause infection by entering the skin through a hair follicle, small scrape, or puncture, although sometimes there is no obvious point of entry.
Filled with pus — a mixture of old and white blood cells, bacteria, and dead skin cells — carbuncles must drain before they’re able to heal. Carbuncles are more likely than boils to leave scars.
An active boil or carbuncle is contagious: the infection can spread to other parts of the person’s body or to other people through skin-to-skin contact or the sharing of personal items. So it’s important to practice appropriate self-care measures, like keeping the area clean and covered, until the carbuncle drains and heals.