A carbuncle is a cluster of boils caused by bacterial infection, most commonly with Staphylococcus aureus or Streptococcus pyogenes. The presence of a carbuncle is a sign that the immune system is active and fighting the infection. 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. In the early 21st century, infection involving methicillin-resistant staphylococcus aureus (MRSA) has become more common. A carbuncle is a cluster of several boils, which is typically filled with purulent exudate (dead neutrophils, phagocitized bacteria, and other cellular components). Fluid may drain freely from the carbuncle, or intervention involving an incision and drainage procedure may be needed.