A 46-year-old patient, after his recovery from flu, has suddenly started suffering from pain in the 36th tooth, the crown of which had been destroyed. The soft tissues surrounding the lower jaw are swollen. There are high fever up to 39°C and chill observed. Objectively: there is prominent swelling of the left lower jaw alveolar mucosa (both oral and vestibular surfaces). The 34th, 35th and 37th intact teeth are mobile. Percussion is painful. The gingival pockets produce purulent exudate. Vincent's symptom is observed. What diagnosis can be assumed?