Parents of a 6-year-old child complain about pain in the child's submandibular region on the left, body temperature rise up to 37,5°C. Objectively: the child's face is asymmetric due to the infiltration of the submandibular region on the left. The infiltration is soft and elastic, mobile, 2x2,5 cm large; its palpation is slightly painful, the skin is unchanged. The teeth are intact. Pharynx is hyperaemic. What is the most likely diagnosis?