Tonsillitis is bacteria infection of the tonsils. It is caused by bacteria usually present in the mouth or from the nostrils and airway.
Prevention is same as prevention of bacteria infections... Good personal hygiene, adequate nutrition and immunization against the bacteria where feasible... HIB AND PCV13 vaccines for examples.
Treatment of upper respiratory tract infections promptly can also prevent spread. Children with enlarged tonsils are prone to Tonsillitis especially the recurrent Tonsillitis and may require tonsillectomy as permanent solution