A carbuncle is a cluster of boils, draining pus onto the skin. It is usually caused by bacterialinfection, most commonly with Staphylococcus aureus or Streptococcus pyogenes, which can turn lethal. However, the presence of a carbuncle is actually a sign that the immune system is working. The infection is contagious and may spread to other areas of the body, or other people; those living in the same residence may develop carbuncles at the same time.