Folliculitis may evolve into a furuncle (“boil”), which is a deeper infection. The term carbuncle refers to an aggregation of furuncles.
Furuncles are painful nodules or abscesses (walled-off collections of pus) that began in an infected hair follicle; they are more common in boys and young adults.
S. aureus is the usual etiologic agent although MRSA is also now a frequent cause.
As with folliculitis, furunculosis is more common in diabetic patients and in obese persons.
A furuncle is a tender, painful subcutaneous nodule with overlying erythema (Fig. 16.15).
Untreated, it may rupture and drain spontaneously (Fig. 16.16) or as it further evolves, a fluctuant abscess may form (Fig. 16.17).
Distribution of Lesions
Can arise anywhere but most often seen in the axillae, inguinal area, posterior neck, thighs, and buttocks (hair-bearing areas and body folds).
A contiguous cluster of furuncles, most often arising on the occipital scalp, is referred to as a carbuncle (Fig. 16.18 see also Fig. 5.8).
Differential Diagnosis 
Management