Tonsillitis happens when your tonsils get infected by a virus or, less commonly, bacteria. Viruses cause up to 70% of cases, while bacteria (most often Group A Streptococcus, the same germ behind strep throat) account for the rest. You catch it the same way you catch a cold: by breathing in respiratory droplets or touching contaminated surfaces, then touching your mouth or nose.
Viral vs. Bacterial Causes
The viruses behind tonsillitis are familiar ones. Cold viruses, flu viruses, and similar respiratory bugs are responsible for the majority of infections. These viral cases typically resolve on their own within a week or so, much like a cold does.
Bacterial tonsillitis is essentially strep throat. Group A Streptococcus is responsible for roughly 20 to 30% of sore throats in children and 5 to 15% in adults. The distinction matters because bacterial tonsillitis requires antibiotics to clear, while viral tonsillitis does not. A rapid strep test or throat swab is the only reliable way to tell the two apart, since they look and feel similar.
How the Infection Spreads
The germs that cause tonsillitis travel through respiratory droplets released when an infected person coughs, sneezes, talks, or laughs. You can also pick them up by sharing food, drinks, or utensils with someone who’s sick, or by kissing. Children in schools and daycares are especially exposed because they’re in close quarters with dozens of other kids throughout the day.
One detail that catches people off guard: a person with tonsillitis is typically contagious one to two days before symptoms even appear. That means you can catch it from someone who seems perfectly healthy. Once someone with bacterial tonsillitis starts antibiotics, they become much less contagious within about 24 hours. Without antibiotics, they can spread the bacteria for a couple of weeks.
What Happens Inside Your Throat
Your tonsils sit at the back of your throat like a pair of immune system sentries. They’re designed to intercept germs entering through your mouth and nose. Unlike most tissue, tonsils have deep, branching pockets called crypts that are specifically built to trap foreign material and deliver it to immune cells waiting underneath. There’s no protective barrier between the surface of these crypts and the immune tissue below, which makes tonsils extremely efficient at detecting threats.
The problem is that this open design also makes tonsils vulnerable. When a virus or bacterium overwhelms the local immune response, the tonsils swell with inflammation as your body floods the area with infection-fighting cells. That’s the redness, swelling, and pain you feel. White patches or spots on the tonsils are pockets of immune cells and debris collecting in those same crypts.
Why Some People Get It Over and Over
Some people seem to get tonsillitis several times a year, and there’s a biological explanation beyond bad luck. Bacteria can form protective communities called biofilms deep inside the tonsil crypts. These biofilms are colonies of bacteria embedded in a sticky, self-made shield that makes them extraordinarily resistant to both antibiotics and your immune system. Research published in JAMA Otolaryngology found biofilms in the tissue and crypts of inflamed tonsils, and demonstrated that bacteria sheltered in a biofilm can withstand antibiotic concentrations thousands of times higher than what would kill the same bacteria floating freely.
The bacteria inside these biofilms don’t actively divide, so they fly under the radar of your immune defenses. But they keep producing toxins that fuel low-grade inflammation. When conditions shift, those bacteria can reactivate and trigger a fresh infection. This cycle of quiet persistence followed by flare-ups is a key reason some people deal with tonsillitis repeatedly, even after completing a full course of antibiotics.
Who’s Most at Risk
Age is the single biggest risk factor. Tonsillitis is overwhelmingly a childhood condition. Bacterial tonsillitis peaks in children between ages 5 and 15, which lines up with the school years when kids are packed together in classrooms, sharing supplies, and not always washing their hands thoroughly. Adults can absolutely get tonsillitis, but it becomes less common with age as the tonsils naturally shrink and the immune system builds a broader catalog of defenses against common pathogens.
Beyond age, any environment that puts you in prolonged close contact with others raises your risk. Think schools, dormitories, military barracks, or crowded workplaces. Exposure to cigarette smoke, frequent upper respiratory infections, and anything that weakens your immune system (poor sleep, high stress, chronic illness) can also tip the odds against you.
How to Lower Your Risk
Since tonsillitis spreads through the same routes as colds and flu, the prevention playbook is straightforward. Wash your hands regularly, especially before eating and after being in public spaces. Avoid sharing cups, water bottles, or utensils. Stay away from people who are visibly sick when possible, and keep in mind that someone can be contagious before they show symptoms.
For parents, teaching young children to cough into their elbow and wash their hands after blowing their nose makes a measurable difference. Replacing toothbrushes after a bout of tonsillitis is also a practical step, since bacteria can linger on bristles. If your child keeps getting tonsillitis (generally defined as seven or more episodes in a single year, or five per year over two years), a conversation about tonsil removal becomes reasonable. That procedure eliminates the tissue where infections take hold and, for kids with truly recurrent infections, tends to dramatically reduce the cycle.