Tonsillitis happens when a virus or bacterium infects your tonsils, triggering inflammation that causes the swelling, pain, and redness you feel at the back of your throat. Viruses cause 70% to 95% of cases, with bacteria responsible for the rest. The process starts with how your tonsils are built and the job they’re designed to do.
Your Tonsils Are Designed to Trap Germs
Your tonsils are clumps of lymphatic tissue positioned at the back of your throat, forming a ring (along with similar tissue near your nose and tongue base) around the opening where air and food enter your body. Their job is to intercept germs before they travel deeper into your respiratory or digestive tract. They’re packed with white blood cells ready to launch an immune response against anything they catch.
The surface of each tonsil is covered in deep folds called crypts. These crypts increase the surface area exposed to whatever you breathe in or swallow, making the tonsils better at sampling and trapping pathogens. But those same crypts also create sheltered pockets where viruses and bacteria can settle, multiply, and eventually overwhelm the local immune defenses. When that happens, the tonsils themselves become infected rather than just filtering the threat. The result is tonsillitis.
How Viruses Cause Most Cases
The majority of tonsillitis infections are viral. Common culprits include rhinoviruses (the usual cause of colds), adenoviruses, respiratory syncytial virus, and coronaviruses. These viruses land on the crypt lining, begin replicating inside the surface cells, and trigger your immune system to flood the area with inflammatory signals and white blood cells. That immune surge is what produces the swelling, redness, and pain.
One virus worth knowing about separately is Epstein-Barr virus, the cause of infectious mononucleosis (mono). EBV has a particular affinity for tonsillar tissue. Once contracted, it can remain dormant in the tonsils for life. When it’s active, it often produces especially severe swelling, sometimes enough to make swallowing or even breathing difficult. Mono-related tonsillitis tends to hit teenagers and young adults hardest and can take weeks to fully resolve, compared to a few days for most other viral causes.
How Bacteria Infect the Tonsils
When tonsillitis is bacterial, group A Streptococcus (the “strep” in strep throat) is the most common cause. It accounts for 15% to 30% of tonsillitis cases in children aged 5 to 15, and 5% to 15% in adults. Other bacteria can occasionally be involved, including one called Fusobacterium necrophorum, which is increasingly recognized in adolescents and young adults.
Strep bacteria are particularly well-equipped to infect tonsillar tissue. They carry at least 11 different surface proteins that help them stick to the cells lining your throat. Once attached, certain proteins (especially M protein) allow the bacteria to push their way inside those cells. The bacteria also have a protective outer capsule that makes them slippery to your immune system’s first responders, the white blood cells that normally engulf and destroy invaders. On top of that, strep bacteria release enzymes that break down surrounding tissue, helping the infection spread, and a protein that blocks the chemical signals your body uses to call more immune cells to the area. This combination of attachment, invasion, and immune evasion is what makes strep tonsillitis feel more aggressive than most viral cases.
How Tonsillitis Spreads
The viruses and bacteria that cause tonsillitis spread through respiratory droplets (coughing, sneezing, talking) and through contact with contaminated surfaces. Sharing drinks, utensils, or close face-to-face contact are common transmission routes, especially among children in schools or daycare. That said, tonsillitis itself doesn’t typically spread as a distinct illness. What spreads is the underlying virus or bacterium, and not everyone who catches it will develop tonsillitis specifically. Some people get a regular sore throat, a cold, or no symptoms at all.
Why Some People Get It Repeatedly
Children between 5 and 15 are the most prone to tonsillitis, partly because their immune systems are still learning to recognize common pathogens and partly because school environments provide constant exposure. But some people, children and adults alike, develop recurrent tonsillitis that keeps coming back several times a year.
The deep crypts in the tonsils can harbor bacteria even after an infection clears, creating a reservoir for reinfection. Over time, repeated infections can scar and reshape the tonsillar tissue, making those crypts deeper and harder for the immune system to patrol effectively. This creates a cycle where each infection slightly increases the likelihood of the next one.
When tonsillitis recurs frequently enough, surgery becomes an option. The widely used Paradise criteria suggest tonsillectomy for children who have seven or more documented episodes in a single year, five or more per year for two consecutive years, or three or more per year for three consecutive years. Adults with similarly frequent episodes are also considered candidates, though the recovery from surgery tends to be more painful and longer for adults than for children.
Telling Viral From Bacterial Tonsillitis
This distinction matters because bacterial tonsillitis (especially strep) benefits from antibiotics, while viral tonsillitis does not. Clinicians use scoring systems to estimate the probability before ordering a test. The most common is the Centor score, which assigns one point each for fever above 100.4°F, swollen and tender neck lymph nodes, white patches or swelling on the tonsils, and the absence of a cough. A score of 0 to 1 means only a 7% to 12% chance of strep. A score of 4 pushes the probability to around 57%.
Viral tonsillitis often comes with other cold symptoms: runny nose, cough, watery eyes, hoarseness. Bacterial tonsillitis tends to arrive more abruptly, with a higher fever, white or yellow patches on the tonsils, and no cough. But there’s enough overlap that a rapid strep test or throat culture is usually needed to confirm bacterial infection before starting antibiotics.
Complications From Untreated Bacterial Tonsillitis
Viral tonsillitis almost always resolves on its own within a week. Bacterial tonsillitis usually does too, but leaving strep untreated carries real risks. The most immediate is a peritonsillar abscess (sometimes called quinsy), a pocket of pus that forms next to the tonsil. This causes intense one-sided throat pain, difficulty opening the mouth, and a muffled voice. It requires drainage and is the most common complication clinicians watch for.
The rarer but more serious concern is rheumatic fever, an inflammatory condition that can damage the heart valves. It affects roughly 0.2% of children with untreated strep infections. Antibiotics given within the first 9 days of a strep infection effectively prevent it. Another uncommon complication, Lemierre’s syndrome, involves infection spreading to the large vein in the neck, potentially sending infected clots to the lungs. This is more associated with Fusobacterium infections in teenagers and young adults.
Post-streptococcal kidney inflammation (glomerulonephritis) is another possible outcome, though notably, antibiotic treatment of the throat infection does not prevent this particular complication. It typically resolves on its own but can cause temporary swelling and changes in urine color.