An estimated 1 to 10% of the cases are caused by Epstein-Barr virus.Tonsillitis can also stem from infection with bacteria, predominantly Group A β-hemolytic streptococci (GABHS), which causes strep throat. Bacterial infection of the tonsils usually follows the initial viral infection. When tonsillitis recurs after antibiotic treatment for streptococcus bacteria, it is usually due to the same bacteria as the first time, which suggests that the antibiotic treatment was not fully effective. Less common bacterial causes include:  Streptococcus pneumoniae, Mycoplasma pneumoniae, Chlamydia pneumoniae, Bordetella pertussis, Fusobacterium sp., Corynebacterium diphtheriae, Treponema pallidum, and Neisseria gonorrhoeae.Anaerobic bacteria have been implicated in tonsillitis, and a possible role in the acute inflammatory process is supported by several clinical and scientific observations.Sometimes tonsillitis is caused by an infection of spirochaeta and treponema, which is called Vincents angina or Plaut-Vincent angina.Within the tonsils, white blood cells of the immune system destroy the viruses or bacteria by producing inflammatory cytokines like phospholipase A2, which also lead to fever. The infection may also be present in the throat and surrounding areas, causing inflammation of the pharynx. Diagnosis
There is no firm distinction between a sore throat that is specifically tonsillitis and a sore throat caused by inflammation in both the tonsils and also nearby tissues. An acute sore throat may be diagnosed as tonsillitis, pharyngitis, or tonsillopharyngitis (also called pharyngotonsillitis), depending upon the clinical findings.
Aerobic and anaerobic beta lactamase producing bacteria that reside in the tonsillar tissues can "shield" group A streptococcus from penicillins. There is no significant difference in efficacy of various groups of antibiotics for treating tonsillitis. Intravenous antibiotics can be for those who are hospitalized with inability to swallow and presented with complications. Oral antibiotics can be resumed immediately if the person is clinically improved and able to swallow orally. Antibiotic treatment is usually taken for seven to ten days. Pain medication
Paracetamol and  nonsteroidal anti-inflammatory drugs (NSAIDs) can be used to treat throat pain in children and adults. Codeine is avoided in children under 12 years of age to treat throat pain or following tonsilectomy. NSAIDs (such as ibuprofen) and opioids (such as codeine and tramadol) are equally effective at relieving pain, however, precautions should be taken with these pain medications. NSAIDs can cause peptic ulcer disease and kidney damage. Opioids can cause respiratory depression in those who are vulnerable. Anaesthetic mouthwash can also be used for symptomatic relief. Corticosteroids
Corticosteroids reduce tonsillitis pain and improve symptoms in 24 to 48 hours. Oral corticosteroids are recommended unless the person is unable to swallow medications. Surgery
When tonsillitis recurs frequently, often arbitrarily defined as at least five episodes of tonsillitis in a year, or when the palatine tonsils become so swollen that swallowing is difficult as well as painful, a tonsillectomy can be performed to surgically remove the tonsils. Children have had only a modest benefit from tonsillectomy for repeated cases of tonsillitis.