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. 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.