The peritonsillar space lies between each tonsil and the wall of the throat. An infection can cause a pus-filled swelling (abscess) to develop in this space. Peritonsillar abscesses, also called quinsy, usually occur as a complication of tonsillitis. They most often are caused by "strep throat" bacteria (group A beta-hemolytic streptococci).
If a peritonsillar abscess is not treated promptly, the infection can spread to the neck, roof of the mouth and lungs. The swelling can push the tonsil closest to it into the center of your throat and move the uvula (the flap of tissue hanging in the back of your throat) from the center toward the unaffected side of your throat. In severe cases, the swelling can make breathing difficult or can close your airway.
Peritonsillar abscesses most often are found in older children, adolescents and young adults. They are less common than in the past because tonsillitis now often is treated with antibiotics, which destroy the infection-causing bacteria.