A carbuncle is a cluster of boils, draining pus onto the skin. It is usually caused by bacterial infection, 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.