A36-year-old member of the national guard who has just returned from iraq consults you because of several "boils" on the back of his neck that have failed to heal over the last 6 months, despite two week-long courses of cephalexin (keflex). you observe three 1- to 2-cm raised minimally tender lesions with central ulceration and crust formation. he denies any fever or systemic symptoms. the most likely cause of these lesions is: