Boils are basically pus-filled nodules and it is also called a furuncle. It is a deep folliculitis which is an infection of the hair follicle. Mostly boils are caused by Staphylococcus aureus commonly found on the skin, nostrils, armpits, between the legs, and in the cleft between the buttocks. Staphylococcus aureus may be transferred to other sites from the nostrils via the fingernails. Sometimes Individual boils are clustered together and make carbuncles.