What is Tonsillitis Tonsillitis is a tonsil inflammation usually caused by a viral or bacterial infection. Streptococcus (Strep bacteria), Adenoviruses, influenza virus, Epstein- Barr virus, parainfluenza virus, enteroviruses, and herpes simplex virus are also common causes of tonsillitis. The tonsillar infection spreads and deeply penetrates nearby tissues. It is more