Tonsillitis is an infection of the tonsils, which are two masses of tissue at the back of the throat that act as filters, trapping germs that could otherwise enter the airways and cause infection. Tonsillitis is usually caused by a viral infection, such as a cold, but it can also be due to a bacterial infection such as strep throat. The most common bacterium causing tonsillitis is Streptococcus pyogenes (group A streptococcus), the bacterium that causes strep throat. Other strains of strep and other bacteria can also cause tonsillitis. The tonsils are the immune systems first line of defense against bacteria and viruses that enter the mouth, which makes them particularly vulnerable to infection and inflammation.

Risk factors for tonsillitis include young age, frequent exposure to germs, and a weakened immune system. Symptoms of tonsillitis include inflamed and swollen tonsils, throat pain or tenderness, fever, red tonsils, a white or yellow coating on the tonsils, painful blisters or ulcers on the throat, headache, loss of appetite, ear pain, trouble swallowing, and swollen glands in the neck or jaw.

Most cases of tonsillitis are treated with self-care measures such as rest, drinking fluids, and taking pain relievers. If the tonsillitis is caused by a bacterial infection, antibiotics may be prescribed. Complications of tonsillitis are rare but can include disrupted breathing during sleep, infection that spreads deep into surrounding tissue, and peritonsillar abscess.