Tonsillitis is caused by a viral infection in 70% to 95% of cases, with the remaining cases typically caused by bacteria. Your tonsils sit at the back of your throat and act as a first line of defense against germs you breathe in or swallow. When a virus or bacterium overwhelms that defense, the tonsils become inflamed, swollen, and painful.
Viral Causes Are the Most Common
The viruses behind most cases of tonsillitis are the same ones that cause colds and other respiratory infections. Rhinovirus (the common cold virus), respiratory syncytial virus, adenovirus, and coronavirus are the most frequent culprits. These viruses are circulating constantly, especially during fall and winter, which is why tonsillitis follows a similar seasonal pattern.
One virus worth knowing about separately is Epstein-Barr virus, which causes mononucleosis (“mono”). EBV-related tonsillitis tends to be more severe and longer-lasting than other viral types. Along with a painfully inflamed throat and swollen neck lymph nodes, mono can cause extreme fatigue, an enlarged spleen, and a rash. Because these symptoms overlap with other infections, EBV often requires a blood test to confirm.
Bacterial Tonsillitis and Strep Throat
When bacteria are the cause, Group A Streptococcus is responsible in the vast majority of cases. This is the same bacterium behind strep throat, and the two conditions overlap significantly. About 30% of children and 10% of adults who show up with a sore, inflamed throat actually have a strep infection. That gap matters: despite the relatively low rates, antibiotics are prescribed to 72% of adults and 56% of children with throat complaints, meaning a large portion of prescriptions target infections that are viral and wouldn’t respond to antibiotics anyway.
Strep bacteria latch onto specific receptor sites on the surface of your tonsils. Once attached, they trigger an aggressive immune response that causes the redness, swelling, and white patches you can sometimes see on infected tonsils. Bacterial tonsillitis is more likely to produce a high fever (above 38°C or 100.4°F), swollen and tender lymph nodes under the jaw, and a noticeable absence of cough or runny nose. Viral tonsillitis, by contrast, often arrives alongside typical cold symptoms like congestion and sneezing.
How It Spreads
Both viral and bacterial tonsillitis spread easily from person to person. The main routes include:
- Inhaling airborne droplets when someone nearby coughs or sneezes
- Kissing or sharing utensils, food, or drinks
- Touching a contaminated surface (a doorknob, phone, or countertop) and then touching your nose or mouth
- Close contact with someone who’s sick, even if they aren’t showing symptoms yet
Children are especially vulnerable because they spend long hours in close quarters at school and daycare, and they’re less consistent about hand hygiene. This is a big reason tonsillitis peaks in childhood and becomes less common in adulthood, when the tonsils naturally shrink and the immune system has encountered more of these pathogens before.
What Happens Inside Your Tonsils
Your tonsils are covered in small pits and folds called crypts. These crypts are designed to trap incoming germs so your immune system can learn to recognize them. Specialized immune cells on the tonsil surface identify the invader and kick off an inflammatory response, sending white blood cells to the area. That flood of immune activity is what causes the swelling, redness, and pain you feel.
In a healthy immune response, this process clears the infection within a few days. But when the infection is severe or keeps returning, the system can start working against itself. In chronic or recurrent tonsillitis, the distribution of immune cells shifts: fewer protective cells remain on the tonsil surface where they’re needed, and more cluster deep in the crypts. This makes the tonsils less effective at catching new threats and more prone to ongoing inflammation.
Non-Infectious Causes of Chronic Tonsillitis
Not every case of tonsillitis traces back to a virus or bacterium. When tonsillitis becomes chronic, meaning the inflammation persists or keeps returning over months, the trigger is often something other than infection. Acid reflux can send stomach acid up into the throat, irritating the tonsils repeatedly. Allergies that cause postnasal drip keep the tonsils bathed in mucus and inflammatory chemicals. Tonsil stones, which form when food particles and bacteria get trapped in the tonsillar crypts and harden into small calcified lumps, can also cause persistent irritation and low-grade inflammation.
Environmental Factors That Raise Your Risk
Certain environmental exposures make your tonsils more vulnerable to infection by irritating the mucous membranes of your throat. Secondhand tobacco smoke is one of the most well-documented: it’s linked to frequent upper respiratory infections in both adults and children, along with chronic throat irritation. Children living in households with smokers are particularly at risk.
Indoor air pollutants beyond tobacco smoke can also play a role. Combustion byproducts from gas stoves, space heaters, and fireplaces release nitrogen dioxide and sulfur dioxide, both of which irritate the throat and nasal passages. Volatile organic compounds from cleaning products, paint, pesticides, and new furniture cause similar upper respiratory irritation. None of these pollutants cause tonsillitis directly, but they weaken the mucosal barrier that normally helps your tonsils do their job, making it easier for viruses and bacteria to gain a foothold.
How Doctors Identify the Cause
Figuring out whether tonsillitis is viral or bacterial matters because only bacterial cases benefit from antibiotics. Doctors use a clinical scoring system that awards points based on four signs: white or yellowish coating on the tonsils, tender swollen lymph nodes in the front of the neck, fever above 38°C, and no cough. A modified version of this score also factors in age, adding a point for children ages 3 to 14 and subtracting one for adults 45 and older, since strep is far more common in younger patients.
These scoring systems help, but they’re imperfect. At strict cutoffs, too many true strep cases get missed. At looser cutoffs, too many people without strep end up on unnecessary antibiotics. That’s why a rapid strep test or throat culture is typically used alongside the clinical score to confirm a bacterial cause before prescribing treatment. If strep is ruled out, the tonsillitis is almost certainly viral, and recovery depends on rest, fluids, and managing symptoms like pain and fever.