A 18-year-old young man complains of bleeding and pain in his gums. The disease onset was 4 days ago. Objectively the skin is pale, body temperature is 38.5°C. Submandibular lymph nodes on the left are enlarged, painful, non-fused with the surrounding tissues. The gingival papillae and gingival margin in the area of 33, 34, 35, 36, and 37 are hyperemic, ulcerated, and covered in necrotic deposit. Teeth are covered with soft dental plaque. Make the diagnosis: