Tonsillitis is inflammation of the tonsils. It's usually caused by a viral infection or, less commonly, a bacterial infection. Tonsillitis is a common condition in children, teenagers and young adults. The symptoms of tonsi Tonsillitis is inflammation of the tonsils. It's usually caused by a viral infection or, less commonly, a bacterial infection. Tonsillitis is a common condition in children, teenagers and young adults. The symptoms of tonsillitis include: a sore throat and pain when swallowing.llitis include: a sore throat and pain when swallowing.