About Tonsillitis: Causes
There are three common causes of tonsillitis:
Group A streptococci - These common bacteria infect the throats of one person in five, including adults. Many people have no symptoms. They can still transmit the bug, however. Group A strep is the most common of all bacteria that can cause disease, and is also responsible for strep throat. It's also capable under some circumstances of causing more serious illness. Group A strep probably accounts for most cases of tonsillitis, and is the kind most likely to cause complications.
Various respiratory viruses, especially cold and flu viruses, account for most of the rest of cases of tonsillitis. These infections can be sometimes milder than bacterial infections.
Infectious mononucleosis, caused by the Epstein-Barr virus, can often include symptoms of inflamed tonsils. Among younger children, the symptoms normally associated with mononucleosis in adults are often absent, but tonsillitis is common.