Carbuncle is a kind of skin infection which involves the group of hair follicles. The area that is infected forms a lump deep under the skin and can contain pus. A condition where you have more than one carbuncle is called carbunculosis.Carbuncles are mostly caused by a certain bacteria known as staphylococcus aureus. Carbuncle is contagious and may spread to other parts of the body. It may spread to other people too.Several skin boils come together to form carbuncle. The lump formed is filled with pus, fluid and dead tissue. The fluid inside the carbuncle may drain out, however in some cases the mass may be so deep that the fluid cannot drain on its own.