Tonsillitis causes

Causes
Bacterial tonsillitis may be caused by Group A streptococcal bacteria, resulting in strep throat. Viral tonsillitis may be caused by numerous viruses such as the Epstein-Barr virus (the cause of infectious mononucleosis) or the Adenovirus.

Sometimes, tonsillitis is caused by a superinfection of spirochaeta and treponema, in this case called Vincent's angina or Plaut-Vincent angina.