Tonsillitis (/ˌtɒnsɪˈlaɪtɪs/ TON-si-LEYE-tis) is inflammation of the tonsils most commonly caused by viral or bacterial infection. Symptoms may include sore throat and fever.When caused by a bacterium belonging to the group A streptococcus, it is typically referred to as strep throat.onsillitis (/ˌtɒnsɪˈlaɪtɪs/ TON-si-LEYE-tis) is inflammation of the tonsils most commonly caused by viral or bacterial infection. Symptoms may include sore throat and fever.