A peritonsillar abscess is most often a complication of tonsillitis. The bacteria involved are similar to those that cause strep throat. Streptococcal bacteria most commonly cause an infection in the soft tissue around the tonsils (usually just on one side). The tissue is then invaded by anaerobes (bacteria that can live without oxygen), which enter through nearby glands. 1  Dental infection (such as periodontitis and gingivitis âgum disorders) may be a risk factor. 2  Chronic tonsillitis. 3  Infectious mononucleosis. 4  Smoking. 5  Chronic lymphocytic leukemia (CLL). 6  Stones or calcium deposits in the tonsils ....