A 27-year-old woman complains of painful and bleeding gums, with the signs aggravating during eating, and indisposition. One week ago she had a case of URTI. Within the last 5 years she periodicaly presents with gingival hemorrhages. Objectively: the gums are bright red, markedly swollen; gingival papillae are friable, bleed on the slightest touch. There is moderate accumulation of dental calculus, large amount of soft dental deposit. Submaxillary lymph nodes are enlarged, painful on palpation. On X-ray: osteoporosis of alveolar septa is observed. What is the most likely diagnosis?