An 11-year-old child complains of pain and bleeding gums when eating, bad breath, weakness and headache that have been lasting for two days. Objectively: gingival mucosa in the area of front lower teeth is swollen, brightly hyperemic, easily bleeds when touched. Gingival papillas are covered with dirty-grey coating; when coating is removed the underlaying surface is bleeding and painful. The occlusion is deep. There are significant deposits of soft dental plaque. The regional lymph nodes are slightly enlarged and painful when palpated. Body temperature is 37,6°C. What is the provisional diagnosis?