A 24-year-old man complains of painful and bleeding gums. The condition onset was 3 days ago after the patient had a case of acute respiratory disease. Objectively the gingival mucosa is swollen, bright-red, bleeds on probing, painful on palpation; tips of the gingival papilla are rounded; soft dental deposit is observed. X-ray shows no changes in the bone. What is the most likely diagnosis?