Swollen tonsils are most often caused by a viral infection, which accounts for up to 70% of tonsillitis cases. The remaining cases are typically bacterial, with Group A Streptococcus (the bacteria behind strep throat) being the most common culprit. But infections aren’t the only trigger. Acid reflux, tonsil stones, environmental irritants, and even sexually transmitted infections can all cause your tonsils to swell.
Viral Infections Are the Most Common Cause
The same viruses that give you the common cold or the flu are responsible for most cases of tonsil swelling. Adenoviruses and rhinoviruses are frequent offenders, but the Epstein-Barr virus deserves special mention. It causes infectious mononucleosis (mono), which is notorious for producing severely swollen tonsils alongside extreme fatigue, fever, swollen lymph nodes in the neck, and sometimes an enlarged spleen. Mono can take weeks to fully resolve, while most other viral cases of tonsillitis clear up within 7 to 10 days on their own.
Viral tonsillitis doesn’t respond to antibiotics. The treatment is rest, fluids, and over-the-counter pain relief to manage the sore throat and fever while your immune system does the work.
Bacterial Infections, Especially Strep
Group A Streptococcus is the most common bacterial cause of throat and tonsil infections, and it primarily affects school-age children between 5 and 15. The bacteria work by latching onto the cells lining your throat using specialized surface proteins. Once attached, they don’t just sit on the surface. They can actually invade the cells of your tonsils and embed themselves inside, which is one reason strep can be stubborn. Roughly 30% of strep throat cases fail to fully clear, and researchers have found strep bacteria living inside tonsillar tissue removed from these patients.
Other bacteria in the same family (Groups C and G streptococci) can also cause tonsillitis and look identical to strep throat on exam, which is why a throat culture or rapid strep test matters for getting the right diagnosis. Bacterial tonsillitis is treated with antibiotics, and symptoms typically improve within a few days of starting them.
How Doctors Tell Viral From Bacterial
One tool clinicians use is the Centor score, a simple checklist of four signs: fever of 100.4°F (38°C) or higher, no cough, swollen lymph nodes at the front of the neck, and visible white patches or swelling on the tonsils. Each sign is worth one point, and a higher score makes a bacterial cause more likely. A low score, especially with a cough present, points toward a virus. This scoring system helps determine whether a strep test is worth running, since most swollen tonsils don’t actually need antibiotics.
Acid Reflux That Reaches Your Throat
A lesser-known cause of tonsil irritation is laryngopharyngeal reflux (LPR), a form of acid reflux where stomach acid travels all the way up into the throat. Unlike typical heartburn, many people with LPR don’t feel the classic burning sensation in their chest. Instead, they notice a persistent sore throat, a feeling of something stuck in the throat, excess mucus, or hoarseness. It only takes a small amount of acid, along with digestive enzymes like pepsin, to irritate the delicate tissue in your throat and tonsils.
LPR also disrupts the normal mucus-clearing process in your throat and sinuses, which means infections linger longer than they should. Many people with LPR assume they have allergies or a cold that won’t go away. Smoke exposure makes it worse, whether you’re the one smoking or you’re regularly around secondhand smoke.
Tonsil Stones
Your tonsils aren’t smooth. They’re covered in small folds and pockets called crypts, and these can trap food debris, bacteria, and dead cells. Over time, that trapped material hardens into small, calcified lumps called tonsil stones. Small stones may go unnoticed, but larger ones can cause localized swelling, difficulty swallowing, a persistent feeling of something stuck in the back of your throat, bad breath, earache, and a sour taste in your mouth. Tonsil stones are not an infection, but they can contribute to chronic irritation and inflammation in the area.
Sexually Transmitted Infections
Tonsil swelling after oral sex is more common than many people realize. Chlamydia and gonorrhea can both infect the throat and tonsils, producing symptoms that look nearly identical to strep throat: redness, pus-like coating on the tonsils, fever, painful swallowing, and swollen neck lymph nodes. In studies of heterosexual adults, chlamydia has been isolated from the throat in roughly 3% to 4% of men and women, and some research suggests it may account for a significant fraction of unexplained throat infections in adults.
The challenge is that it’s essentially impossible to distinguish an STI-related tonsil infection from a standard bacterial one based on appearance alone. Standard strep tests will come back negative, and unless a clinician specifically tests for these organisms, the real cause can be missed entirely. If you have tonsillitis that doesn’t respond to typical treatment, especially after oral sexual contact, specific testing is worth requesting.
When Swelling Becomes Dangerous
A peritonsillar abscess is the most serious complication of tonsil swelling. It develops when infection spreads beyond the tonsil itself and forms a pocket of pus in the tissue beside it. The key warning signs are distinct from ordinary tonsillitis: pain that’s clearly worse on one side and keeps getting worse over days, increasing difficulty swallowing (sometimes to the point where you can’t swallow your own saliva), a muffled or “hot potato” voice, difficulty opening your mouth, ear pain on the affected side, and foul breath.
On examination, the tonsil on the affected side gets pushed downward and toward the center of the throat, and the uvula swells and shifts to the opposite side. A visible bulge forms on the soft palate. This condition requires prompt medical treatment, as it won’t resolve on its own and can progress to more widespread infection if left untreated.
Environmental Irritants and Allergies
Cigarette smoke, air pollution, and strong chemical fumes can all irritate and inflame tonsil tissue without any infection being present. Postnasal drip from allergies can have a similar effect, as mucus constantly draining over the tonsils causes chronic low-grade irritation. If your tonsils seem mildly swollen much of the time without the sharp pain and fever of an acute infection, an environmental or allergic trigger is worth considering, particularly if symptoms follow seasonal patterns or worsen in certain environments.