Causes

Tonsillitis is most often caused by common viruses, but bacterial infections also can be the cause.

The most common bacterium causing tonsillitis is Streptococcus pyogenes (group A streptococcus), the bacterium that causes strep throat. Other strains of strep and other bacteria also may cause tonsillitis.

Why do tonsils get infected?

The tonsils are the immune system's first line of defense against bacteria and viruses that enter your mouth. This function may make the tonsils particularly vulnerable to infection and inflammation. However, the tonsil's immune system function declines after puberty — a factor that may account for the rare cases of tonsillitis in adults.