Close-up of the mouth of a 15 year old female patient with tonsillitis. Her tonsils (on either side of the throat) are inflamed and pussy. Tonsillitis is a bacterial or viral infection that causes a severe sore throat, often accompanied by fever. The tonsils are masses of lymphoid tissue. They are part of the body's immune system and help to protect the throat and upper respiratory tract from infections. This is viral tonsillitis and is treated with painkillers. In recurrent cases the tonsils may be removed.