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.