Causes of tonsillitis. Most cases of tonsillitis are caused by a viral infection. Viruses known to cause tonsillitis include: 1  rhinoviruses – which are viruses that cause the common cold. 2  the influenza virus. 3  parainfluenza virus – which causes laryngitis and croup.4  enteroviruses – which cause hand, foot and mouth disease.iruses known to cause tonsillitis include: 1  rhinoviruses – which are viruses that cause the common cold. 2  the influenza virus. 3  parainfluenza virus – which causes laryngitis and croup.