Definition. Peritonsillar abscess is a bacterial infection. It develops in the back of the throat, behind or above the tonsils. The infection causes a pocket of pus to form. The tonsil area becomes tender. Fever, trouble swallowing, jaw spasms, and bad breath may also occur. This type of abscess usually happens on one side of the throat or the other. The condition is more common in young adults. It can also occur in children. The Tonsils.