Tonsillitis spreads through respiratory droplets and direct contact with an infected person. When someone with a viral or bacterial infection talks, coughs, or sneezes, they release tiny droplets containing the pathogen. You can get sick by breathing those droplets in, touching a contaminated surface and then touching your mouth or nose, or sharing utensils, glasses, or plates with someone who’s infected.
Viruses Cause Most Cases
Up to 70% of tonsillitis cases are caused by viruses, including the same ones responsible for the common cold and flu. These viruses enter through your mouth or nose and reach the tonsils, which sit at the back of your throat and act as a first line of defense against airborne pathogens. Your tonsils trap and filter bacteria and viruses before they can move deeper into your respiratory or digestive tract. When the immune system fights back against a pathogen lodged in the tonsils, it triggers inflammation, and that inflammation is what you experience as tonsillitis.
The remaining cases are bacterial. Group A Streptococcus is the most common bacterial culprit, and when it infects the tonsils, the result is what most people call strep throat. Bacterial tonsillitis from strep tends to hit children between ages 5 and 15, while viral tonsillitis is more common in younger kids.
How the Infection Spreads
Group A strep bacteria live in the nose and throat. Infected people spread them simply by talking, coughing, or sneezing, which sends respiratory droplets into the air. You can catch the infection in three main ways:
- Breathing in droplets released by someone nearby who’s infected
- Touching contaminated surfaces and then touching your mouth or nose
- Sharing items like drinking glasses, utensils, or plates with an infected person
Viral tonsillitis spreads the same way. Cold and flu viruses are highly contagious in close quarters, which is why outbreaks cluster in schools, daycare centers, and households, particularly during winter months when people spend more time indoors together.
Why Children Get It More Often
Children are far more likely to develop tonsillitis than adults, and the reasons are partly behavioral and partly biological. School-age kids interact closely in classrooms, share supplies, and touch shared surfaces constantly, all of which increases their exposure to circulating infections. Their immune systems are also still learning to recognize and fight off common pathogens, making them more susceptible to each new virus or bacterium they encounter.
Tonsil tissue in children tends to be larger and more active than in adults. The tonsils are part of the immune system and are designed to sample pathogens entering through the mouth, but that very function puts them in constant contact with infectious agents. As children grow into adulthood, the tonsils naturally shrink and become less prominent, which is one reason tonsillitis becomes less common with age.
What Happens Inside the Tonsils
Your tonsils are covered by a protective layer of tissue called epithelium. As long as that lining stays intact, it acts as a barrier against infection. But when a pathogen damages or disrupts this lining, bacteria or viruses can penetrate deeper into the tonsil tissue, triggering the immune response that leads to swelling, redness, and pain. In more severe cases, the damaged areas fill with pus.
The tonsils also have deep folds called crypts, and these crypts are especially vulnerable to colonization. Bacteria can settle into these pockets and, once established, begin building structures called biofilms. A biofilm is essentially a protective shell made of sugars that bacteria produce around themselves. Inside this shell, bacteria are shielded from your immune system and, critically, from antibiotics. This is a key reason why some tonsillitis infections keep coming back even after treatment.
Why Some People Get Tonsillitis Repeatedly
Recurrent tonsillitis has puzzled doctors for years. Even when the bacteria causing an infection are fully susceptible to antibiotics in a lab setting, about 20% of strep tonsillitis cases still test positive for bacteria after a complete course of treatment. The biofilm explanation helps make sense of this. Bacteria embedded in biofilms within tonsil crypts can be extraordinarily resistant. In one striking example, a strain of bacteria that could be killed by a low dose of antibiotic in liquid suspension survived antibiotic concentrations 2,500 times higher when growing as a biofilm.
While protected inside these biofilms, bacteria continue producing toxins that irritate the surrounding tissue, keeping inflammation going at a low level. Then, when conditions shift (your immune system dips during another illness, for instance), the bacteria can become active again and trigger a full new episode. This cycle of quiet persistence followed by flare-ups is what makes chronic tonsillitis so frustrating. Research examining tonsils removed from patients with recurrent infections found bacterial biofilms in the crypts of the vast majority of specimens, suggesting this mechanism is common rather than rare.
Other Factors That Raise Your Risk
Beyond close contact with infected people, a few environmental factors make tonsillitis more likely. Exposure to secondhand cigarette smoke irritates the throat lining and can make the tonsils more vulnerable to infection. Air pollution has a similar effect, weakening the mucosal barrier that normally keeps pathogens out. Seasonal timing matters too: most viral and bacterial throat infections peak in fall and winter, when indoor crowding and dry air create ideal conditions for transmission.
Frequent exposure to new groups of people also plays a role. Starting at a new school or daycare, traveling, or working in a setting with lots of close contact all increase the chance of encountering a pathogen your immune system hasn’t seen before. This is why tonsillitis often clusters at the beginning of school terms when children from different households mix together for the first time in weeks.
Viral vs. Bacterial: How They Differ
Knowing whether your tonsillitis is viral or bacterial matters because the treatment is completely different. Viral tonsillitis typically comes with cold symptoms like a runny nose, cough, and hoarseness alongside the sore throat. It resolves on its own as your immune system clears the virus, usually within a week or so.
Bacterial tonsillitis, especially strep, tends to come on more suddenly. The sore throat is often severe, sometimes with white patches or streaks of pus on the tonsils, swollen lymph nodes in the neck, fever, and notably no cough. Doctors use a rapid strep test or throat culture to confirm bacterial infection before prescribing antibiotics. This distinction is important because antibiotics do nothing for viral infections and using them unnecessarily contributes to resistance, making future bacterial infections harder to treat.