Tonsillitis is an infection of the tonsils which is caused by either a virus or a bacterium. There are two tonsils, situated on either side of the back of the throat and they form part of the body's immune system. Tonsillitis is extremely common in children and young people but it can occur at any age. There are three chief types of tonsillitis: acute, subacute and chronic. Acute tonsillitis can either be bacterial or viral in origin. Subacute tonsillitis is caused by the bacterium Actinomyces. Chronic tonsillitis, can last for long periods if untreated, is almost always bacterial. It may also lead to chronic cryptic tonsillitis. Cryptic tonsillitis contains small pockets and pits called crypts. These crypts may trap food particles and form small white-colored growths called tonsilliths.