Most tonsil infections and abscesses in children are caused by bacteria known as Streptococcus agalactiae or "strep". Infection with streptococcus bacteria causes the common and well known "strep throat".
Strep tonsillitis can have serious complications and should be treated as soon as possible. This is because two important illnesses, rheumatic fever affecting the heart and nephritis affecting the kidneys, may result as a complication of tonsillitis. Both are serious conditions in children.
Other possible complications of streptococcal tonsillitis include scarlet fever, sinusitis, pneumonia, and ear infections.
The viruses that cause the common cold or influenza (flu) may be responsible for tonsillitis.
Rapid enlargement of the tonsils can result from an infection called infectious mononucleosis commonly known as glandular fever. Infectious mononucleosis is caused by the Epstein-Barr virus. This virus also affects the adenoids and commonly the lymph nodes of the neck in mononucleosis.
In young children, mononucleosis is usually mild, and can be mistaken for a common cold or flu. When infectious mononucleosis occurs after childhood, the disease may be extremely serious. It may affect the liver and spleen and may lead to jaundice.