Most common cause of parapharyngeal abscess is

Infection of tonsillar fossa leads to the parapharyngeal abscess as a delayed complication of tonsillectomy. Infection of parapharyngeal space can occur from teeth, ear and other spaces like parotid and retropharyngeal spaces. In spite of it being the smallest space in relation to the pharynx, it is the most commonly infected space of the pharynx as infections from tonsil(most commonly), dental, parotid, retropharyngeal, preveebral and submandibular space can all lead to parapharyngeal abscess. (Ref: Scott Brown, 8th edition, VOL 3, Pg no. 551)