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. Therefore, you should practice appropriate self-care measures, like keeping the area clean and covered, until the carbuncle drains and heals.
Carbuncles require medical treatment to prevent or manage complications, promote healing, and minimize scarring. Contact your doctor if you have a boil or boils that have persisted for more than a few days.