Tonsils swell because they are doing their job. These small lumps of tissue at the back of your throat are part of your immune system, and when a virus, bacterium, or other irritant shows up, they often enlarge as they mount a defense. The most common reason is a straightforward infection, but the causes range from the mundane to the serious, and a few patterns of swelling deserve prompt medical attention.
What Tonsils Actually Do
Your palatine tonsils sit at the crossroads where air and food pass through, making them one of the first points of contact between your body and whatever you breathe in or swallow. They act as immune sentinels, sampling incoming germs and triggering defensive responses.1PubMed. Novel Findings on the Development and Immunological Functions of Palatine Tonsils They are part of a ring of lymphoid tissue called Waldeyer’s ring, which includes the adenoids higher up and a patch of tissue at the base of the tongue. Together, these structures form an early-warning network for your mucosal immune system.2PubMed Central. Anatomy and physiology of the palatine tonsils, adenoids, and lingual tonsils
Because tonsils are packed with immune cells, they tend to enlarge whenever they are actively fighting something. In children, that immune activity is especially high, and the tonsils are proportionally larger relative to the throat. By adulthood, they typically shrink. So a swollen tonsil in a child is common and often benign, while in an adult it may warrant closer attention, especially if only one side is affected.
Viral Infections Are the Most Common Culprit
The majority of sore throats with swollen tonsils are caused by viruses. Rhinoviruses (the common cold), adenoviruses, influenza, and respiratory syncytial virus can all trigger tonsillar swelling, often with a runny nose, cough, and general achiness. These infections tend to resolve on their own within a week or so, and antibiotics do nothing for them.
One viral cause worth knowing by name is infectious mononucleosis, commonly called mono. It is caused by Epstein-Barr virus and tends to hit teenagers and young adults hardest. Mono can cause dramatic tonsillar swelling, sometimes so severe that the tonsils nearly touch in the middle of the throat. In one documented case, a patient with acute mono developed bilateral tonsils that crossed the midline, along with pus on the surface and swollen lymph nodes under the jaw.3PubMed Central. A case of suppurative tonsillitis as a complication of acute infectious mononucleosis Beyond the sore throat, mono often brings prolonged fatigue, fever, and an enlarged spleen, and it can take weeks to fully recover. If you are given antibiotics (particularly amoxicillin) while unknowingly infected with Epstein-Barr virus, you may develop a widespread rash, which is a classic diagnostic clue.
Bacterial Infections and Strep Throat
Strep throat is caused by the bacterium Streptococcus pyogenes, also known as group A streptococcus.4Strep Throat. Strep Throat: Public Education It typically comes on fast: a sudden sore throat, pain when swallowing, fever, and red or swollen tonsils that may have white patches of pus. Unlike viral sore throats, strep usually does not cause a cough or runny nose. That absence of cold symptoms is actually one of the clues doctors use when deciding whether to test for it.
Strep matters for two reasons beyond the immediate misery. First, untreated strep can lead to rheumatic fever, which can damage the heart valves. Second, it can trigger post-streptococcal glomerulonephritis, a kidney problem. These complications are uncommon in countries where antibiotics are readily available, but they are the reason doctors test for strep and treat confirmed cases with penicillin or amoxicillin rather than taking a wait-and-see approach.
How reliably can a doctor tell strep from a virus just by looking? Not very. Clinical scoring systems like the Centor score try to predict the likelihood of strep based on symptoms such as fever, tonsillar exudates, swollen lymph nodes, and the absence of a cough. A systematic review found that the Centor score at a threshold of 3 has a sensitivity under 50%, meaning it misses more than half of actual strep cases.5Clinical Microbiology and Infection. Systematic review and meta-analysis of the accuracy of McIsaac and Centor score in patients presenting to secondary care with pharyngitis In children specifically, one study found the score performed even worse, with a sensitivity of just over 22% against rapid antigen testing, leading researchers to conclude that microbiological testing remains necessary for accurate diagnosis.6PubMed Central. Centor scores associated poorly with rapid antigen test findings in children with sore throat The takeaway: if strep is suspected, a throat swab is the way to confirm it. Eyeballing the throat alone is not reliable.
The Dangerous Bacterial Infection Most People Have Never Heard Of
Fusobacterium necrophorum is a bacterium that can cause tonsillitis in otherwise healthy young adults and teenagers. Most of the time it behaves like a run-of-the-mill sore throat, but in rare cases it progresses to Lemierre’s syndrome, a potentially life-threatening condition. What happens is that the infection spreads from the tonsil to the internal jugular vein in the neck, causing a blood clot infected with bacteria. From there, septic clots can break off and travel to the lungs or other organs.7PubMed Central. Human infection with Fusobacterium necrophorum (Necrobacillosis), with a focus on Lemierre’s syndrome
Patients with Lemierre’s syndrome typically start with what looks like garden-variety tonsillitis, then develop worsening one-sided neck pain, high fevers, and rigors (violent shaking chills) that seem out of proportion to a simple sore throat.8PubMed. Lemierre’s syndrome due to Fusobacterium necrophorum The key red flag is a sore throat that initially seems to improve and then sharply worsens, especially with spiking fevers and neck stiffness or swelling along one side. This condition is rare but serious, and it disproportionately strikes people in their teens and twenties.9PubMed Central. Fusobacterium necrophorum sepsis after tonsillitis/pharyngitis
Peritonsillar Abscess
A peritonsillar abscess, sometimes called quinsy, is a pocket of pus that forms in the tissue right next to a tonsil. It is one of the more common complications of tonsillitis and usually affects one side. If you open your mouth wide and look in the mirror, you might see that one side of the soft palate is bulging or pushed forward. The classic symptoms are difficulty opening the mouth fully (trismus), a muffled or “hot potato” voice, severe pain when swallowing, and ear pain on the same side.10PubMed Central. Peritonsillar swelling is not always quinsy
Quinsy needs treatment. It is usually drained with a needle or a small incision, and antibiotics are given alongside. Left untreated, the abscess can spread deeper into the neck or, in extreme cases, compromise the airway. If you have a sore throat that has been getting worse for several days, pain is markedly worse on one side, and you are having trouble opening your mouth or swallowing your own saliva, get medical attention soon rather than waiting it out.
When Only One Tonsil Is Swollen
Swelling on both sides is usually a sign of a systemic infection, like a cold, mono, or strep. Swelling on just one side is more ambiguous and sometimes more concerning. One-sided tonsillar enlargement can result from infection, a localized inflammatory response, or a tumor.11PubMed. Unilateral tonsillar enlargement In many cases, the explanation turns out to be a peritonsillar abscess or a recent infection that happened to affect one side more than the other. But the possibility of a neoplasm makes doctors take one-sided enlargement more seriously, particularly in adults.
In a study of adults who had a tonsillectomy specifically because one tonsil was persistently enlarged, about 23% turned out to have a malignancy on histological examination.12PubMed. Role of tonsillectomy in histology for adults with unilateral tonsillar enlargement That does not mean one in four people with a swollen tonsil has cancer. The patients in that study were referred specifically because their enlargement was persistent and clinically suspicious enough to warrant surgery. Still, it underscores why a tonsil that stays enlarged for weeks after other symptoms have cleared, or that enlarges without any obvious infection, should be evaluated by a doctor. Additional red flags that raise concern for tonsillar malignancy include unexplained weight loss, a lump in the neck, persistent ear pain on one side, and difficulty swallowing that gets worse over time rather than better.
HPV and Tonsillar Cancer
Over the past two decades, human papillomavirus (HPV) has emerged as a major driver of oropharyngeal cancers, particularly squamous cell carcinomas of the tonsil and the base of the tongue. The incidence of HPV-related tonsillar cancer has been rising, even as smoking-related head and neck cancers have declined. One challenge with HPV-positive tonsillar tumors is that the primary growth in the tonsil can be small or even hidden on imaging, while the first noticeable sign is a painless lump in the neck from a metastatic lymph node.13PubMed Central. Radiographic imaging of human papillomavirus related carcinomas of the oropharynx
This means that a persistent neck lump in someone without obvious infection, even if the tonsil itself does not look dramatically swollen, can be a clue to tonsillar cancer. HPV-positive oropharyngeal cancers do tend to respond better to treatment than their HPV-negative counterparts, but catching them early still matters. The HPV vaccine, originally promoted for its role in preventing cervical cancer, is also expected to reduce rates of these oropharyngeal cancers over time, though the full effect will take decades to play out as vaccinated cohorts age.
Non-Infectious Causes of Tonsil Swelling
Not every swollen or irritated tonsil is infected. Tonsil stones (tonsilloliths) are hard, calcified deposits that form in the deep pits and folds of the tonsil surface. Small ones are usually harmless and go unnoticed, but larger ones can cause a feeling of something stuck in the throat, bad breath, and mild pain.14PubMed Central. World’s Largest Tonsillolith Removal: A Case Report With Literature Review of Large Tonsil Stones on Record They can also make the tonsil look asymmetric or slightly enlarged. Most tonsil stones can be dislodged at home with gentle pressure or saltwater gargling, though recurring large stones sometimes become a reason to consider tonsillectomy.
Acid reflux is another underappreciated contributor. Laryngopharyngeal reflux (LPR), where stomach acid reaches the throat, can cause chronic irritation and swelling of the tonsils that mimics infection. A study found a significant association between chronic tonsillitis and LPR, and showed that treating the reflux with acid-suppressing medication and dietary changes improved both the reflux symptoms and the tonsillar inflammation.15PubMed Central. Prevalence and clinical features of laryngopharyngeal reflux in adults with chronic tonsillitis If your tonsils seem perpetually red or swollen but cultures keep coming back negative, reflux is worth investigating, especially if you also have throat clearing, a sensation of a lump in your throat, or a hoarse voice.
Recurrent Fever Syndrome in Children
Some children develop a pattern of tonsil swelling that recurs like clockwork every few weeks, accompanied by high fever, mouth ulcers, and swollen neck glands. This is called PFAPA syndrome (periodic fever, aphthous stomatitis, pharyngitis, and adenitis), and it is the most common periodic fever condition in children, with most cases appearing by age five.16PubMed Central. Periodic fever, aphthous stomatitis, pharyngitis, and adenitis (PFAPA) syndrome: A review Between episodes, the child is completely well. The fevers are typically high and last three to six days, then vanish until the next cycle.
PFAPA is not caused by an infection, and antibiotics do not help. A single dose of a corticosteroid can abort an episode, though it sometimes shortens the interval to the next one. For children with very frequent episodes, tonsillectomy has been shown to resolve or dramatically reduce flares in many cases. The condition tends to burn out on its own eventually, but the years of high fevers and missed school can be disruptive enough that families and doctors weigh intervention earlier.
Swollen Tonsils and Breathing Problems in Children
In young children, tonsillar and adenoid enlargement is the leading cause of obstructive sleep apnea. The tonsils and adenoids are at their most active and largest between roughly ages two and seven, and in some children the tissue grows large enough to partially block the airway during sleep. Classic signs include loud snoring, pauses in breathing during sleep, mouth breathing, restless sleep, and daytime sleepiness or behavioral issues that can be mistaken for ADHD. Research has documented that tonsillar hyperplasia causing obstructive sleep apnea can lead to retained carbon dioxide at night, slowed growth, and impaired physical and psychological development.17JAMA Otolaryngology–Head & Neck Surgery. Tonsillar Hyperplasia in Children: A Cause of Obstructive Sleep Apneas, CO2 Retention, and Retarded Growth
Adenotonsillectomy (removal of both tonsils and adenoids) is the first-line treatment for pediatric obstructive sleep apnea when enlarged tonsils and adenoids are the cause. One thing parents sometimes do not anticipate is that other lymphoid tissue in the ring can compensate by growing after surgery. There is a documented case of a child who had successful adenotonsillectomy but developed obstructive sleep apnea again four years later because the tubal tonsils (near the Eustachian tubes) had enlarged to fill the gap.18Journal of Nippon Medical School. An 8-year-old Girl with a Recurrence of Obstructive Sleep Apnea Syndrome Caused by Hypertrophy of Tubal Tonsils 4 Years after Adenotonsillectomy This is uncommon, but it highlights that follow-up matters.
When Tonsillectomy Is on the Table
For recurrent throat infections, the decision to remove tonsils follows fairly specific guidelines. In children, the American Academy of Otolaryngology recommends considering tonsillectomy when there have been at least seven documented episodes in one year, at least five per year over two years, or at least three per year over three years. Each episode should be documented in the medical record with at least one of the following: fever above 38.3°C (101°F), swollen cervical lymph nodes, tonsillar exudate, or a positive strep test. Below those thresholds, watchful waiting is recommended.19PubMed. Clinical Practice Guideline: Tonsillectomy in Children (Update)
For adults, the guidelines are less rigid, but the same general principle applies: surgery is reserved for people whose recurrent infections are frequent and severe enough to meaningfully affect their quality of life. Other indications for tonsillectomy include obstructive sleep apnea caused by enlarged tonsils, suspicion of malignancy (especially with unilateral enlargement in an adult), and PFAPA syndrome that does not respond to medical management.
One concern parents sometimes raise is whether removing the tonsils will weaken the immune system. A study comparing children who had undergone tonsillectomy with those who had not found that the tonsillectomy group had significantly lower serum levels of the antibodies IgA, IgM, and IgG, while showing no significant difference in key immune cell markers.20PubMed Central. Long-term impacts of tonsillectomy on children’s immune functions What that means in practical terms is still debated. The remaining lymphoid tissue in Waldeyer’s ring and elsewhere in the body takes over much of the tonsils’ surveillance role, and most children who have their tonsils removed do not experience noticeably more infections afterward. Still, it is one reason surgeons try to reserve the procedure for cases that clearly meet the threshold rather than performing it routinely.
Red Flags That Call for Urgent Evaluation
Most sore throats with swollen tonsils resolve on their own or with a short course of antibiotics. But certain warning signs mean you should not wait.
- Difficulty breathing or drooling: Tonsils that are large enough to obstruct the airway are an emergency, especially in a young child. Drooling because swallowing is too painful or physically blocked is also a warning sign of something more than routine tonsillitis.
- One-sided neck swelling or stiffness: Swelling along one side of the neck, especially with spiking fevers, could indicate a deep neck space infection or the early stages of Lemierre’s syndrome.
- Inability to open the mouth: Trismus (lockjaw-like restriction) alongside a sore throat is a hallmark of peritonsillar abscess.
- Persistent unilateral enlargement: A tonsil that stays swollen for weeks without other symptoms, or that keeps growing, needs examination to rule out lymphoma or squamous cell carcinoma.
- Unexplained weight loss or night sweats: Combined with tonsillar swelling, these are classic “B symptoms” that raise concern for lymphoma.
- A painless neck lump with no obvious infection: In the context of HPV-related oropharyngeal cancer, the first sign can be a firm, non-tender lymph node rather than throat pain.
Sore throats are overwhelmingly caused by self-limited infections, and the vast majority of swollen tonsils return to normal within a week or two. But the patterns described above are the ones worth recognizing, because the difference between a routine sore throat and something more serious often comes down to details: which side is swollen, how long it has lasted, what other symptoms are along for the ride, and whether the trajectory is getting better or getting worse.