A peritonsillar abscess is a bacterial infection that usually begins as a complication of untreated sterp throat or tonsillitis. It generally involves a pus _filled pocket that forms near one of your tonsils. Peritonsillar abscesses are most common in children adolescents,and young adults. They aften occur at beginning or end of the winter season, when illnesses like strep throat and tonsillitis are most widespread. Antibiotics are the most common from of treatment.