Tonsillitis is inflammation of the tonsils in the upper part of the throat. It can be acute or chronic. Acute tonsillitis typically has a rapid onset. Symptoms may include sore throat, fever, enlargement of the tonsils, trouble swallowing, and enlarged lymph nodes around the neck. Complications include peritonsillar abscess.

Tonsillitis is most commonly caused by a viral infection and about 5% to 40% of cases are caused by a bacterial infection. When caused by the bacterium group A streptococcus, it is classed as streptococcal tonsillitis also referred to as strep throat. Rarely bacteria such as Neisseria gonorrhoeae, Corynebacterium diphtheriae, or Haemophilus influenzae may be the cause. Typically the infection is spread between people through the air. A scoring system, such as the Centor score, may help separate possible causes. Confirmation may be by a throat swab or rapid strep test.

Treatment efforts involve improving symptoms and decreasing complications. Paracetamol (acetaminophen) and ibuprofen may be used to help with pain. If strep throat is present the antibiotic penicillin by mouth is generally recommended. In those who are allergic to penicillin, cephalosporins or macrolides may be used. In children with frequent episodes of tonsillitis, tonsillectomy modestly decreases the risk of future episodes.

About 7.5% of people have a sore throat in any three-month period and 2% of people visit a doctor for tonsillitis each year. It is most common in school-aged children and typically occurs in the colder months of autumn and winter. The majority of people recover with or without medication. In 82% of people, symptoms resolve within one week, regardless if bacteria or viruses were present. Antibiotics probably reduce the number of people experiencing sore throat or headache, but the balance between modest symptom reduction and the potential hazards of antimicrobial resistance must be recognised.