Tonsillitis in the throat of a 17 year old female patient. Tonsillitis is inflammation of the tonsils, two oval-shaped pads of tissue at the back of the throat on each side. It is usually caused by a viral or less commonly, a bacterial infection. It causes a sore throat, fever and difficulty with swallowing. Treatment typically involves reducing symptoms with painkillers, rest and drinking fluids. If it is caused by a bacterial infection then antibiotic drugs may be required. If it is recurrent and severe, then the tonsils may need to be surgically removed (tonsillectomy).