A 23-year-old man complains of acute gingival bleeding and unpleasant smell from the mouth that appeared 5 days ago. Objectively gingival papillae and marginal gingiva are friable, bright red, swollen, painful, and bleed profusely on palpation. Gingival pockets are 3 mm deep. X-ray shows marked osteoporosis of the interalveolar septa, periodontal ?ssure in the apical areas of the interalveolar septa is enlarged. Cortical plate is intact. Make the diagnosis: