Your tonsils get infected because their job is to get exposed to germs. These two lumps of tissue at the back of your throat act as a first line of immune defense, sampling everything you breathe in and swallow. When a virus or bacterium overwhelms that defense, the tonsils themselves become the site of infection. Viruses cause 70% to 95% of tonsillitis cases, with bacteria responsible for the rest.
Why Tonsils Are Vulnerable by Design
Tonsils sit at the entrance to your throat, positioned to intercept airborne and ingested material before it reaches your lungs or digestive system. Unlike deeper parts of your digestive tract, the mouth has no enzymes breaking down what passes through, so pathogens arrive at the tonsils in their full, active form.
The surface of each tonsil is riddled with deep, branching pockets called crypts. These crypts dramatically increase the tonsil’s surface area, giving the immune system more territory to detect threats. Immune cells inside the crypt lining grab samples of bacteria, viruses, and other foreign particles, then present them to the rest of the immune system so it can learn to fight back. This is useful when things go right. But all that extra surface area also means more places for pathogens to take hold. The same crypts that help your immune system sample germs can trap food debris, dead cells, and bacteria, creating an environment where infection can develop.
How Germs Reach Your Tonsils
The pathogens that cause tonsillitis travel the same routes as cold and flu viruses. When someone with an infection coughs or sneezes, they release tiny respiratory droplets into the air. Breathing those in delivers germs directly to the back of your throat, right where your tonsils sit. You can also pick up the germs by touching a contaminated surface, like a doorknob or shared cup, and then touching your face, nose, or mouth.
Once a pathogen lands on the tonsillar surface, it has a few options. It can multiply on the surface, settle into one of the crypts, or in some cases invade directly into the tissue. What happens next depends on whether it’s a virus or a bacterium.
Viral Tonsillitis: The Most Common Type
The vast majority of tonsil infections are caused by viruses, particularly the same ones responsible for the common cold and influenza. Adenoviruses, the Epstein-Barr virus (which causes mono), and various respiratory viruses all target the throat and can inflame the tonsils. The infection triggers your immune system to flood the area with white blood cells and inflammatory chemicals, which is what causes the swelling, redness, and pain you feel.
Viral tonsillitis typically resolves on its own within a week to ten days. Antibiotics do nothing against viruses, so treatment focuses on managing symptoms: pain relief, fluids, and rest.
How Bacteria Invade Tonsillar Tissue
Bacterial tonsillitis is less common but tends to be more severe. Group A Streptococcus, the bacterium behind strep throat, is the primary culprit. It accounts for 15% to 30% of tonsillitis cases in children aged 5 to 15, and 5% to 15% of cases in adults.
Strep bacteria are particularly aggressive colonizers. They attach to the mucosal lining of the throat and tonsils, form biofilms on the tonsillar surface and inside the crypts, and can even invade individual epithelial cells to survive inside them. Once inside your cells, the bacteria are shielded from many of your body’s defenses. Strep also actively sabotages the immune response. It produces a protein that breaks down chemical signals your body uses to recruit infection-fighting white blood cells called neutrophils to the area. By cutting off these recruitment signals, the bacteria buy themselves time to establish a deeper foothold before your immune system can mount a full response.
This combination of hiding inside cells and suppressing immune signaling is what makes strep throat more than just a sore throat. It’s a bacterial infection that has evolved specific tools to outmaneuver tonsillar defenses.
Why Children Get Tonsillitis More Often
Tonsillitis peaks in school-aged children, roughly between ages 5 and 15. Several factors converge in this age group. Their immune systems are still learning to recognize common pathogens, so they’re more susceptible to the viruses and bacteria circulating in classrooms and playgrounds. Children also tend to share objects, touch their faces frequently, and spend long hours in close quarters with other kids, all of which increase exposure.
Tonsils themselves are proportionally larger in children and begin to shrink after puberty. By adulthood, the tonsils have processed years of immune challenges and are less reactive. This is one reason tonsillitis becomes less common with age, though adults certainly still get it.
How Doctors Tell Viral From Bacterial
Distinguishing between viral and bacterial tonsillitis matters because only bacterial cases benefit from antibiotics. Doctors use a clinical scoring system that evaluates five factors: the patient’s age, whether they have swollen lymph nodes in the neck, the presence or absence of a cough, fever, and whether the tonsils have white or yellow patches (exudate).
Each factor adds or subtracts a point. Low scores (0 or 1) suggest a viral cause and mean no testing or antibiotics are needed. A score of 2 or 3 may prompt a rapid strep test or throat culture. Scores of 4 or 5 indicate a higher probability of strep, and antibiotics may be started based on symptoms even before test results return. A cough, notably, points toward a viral cause, since strep throat rarely produces one.
When Infection Spreads Beyond the Tonsils
Most tonsil infections, whether viral or bacterial, resolve without complications. But untreated bacterial tonsillitis, particularly strep, can lead to problems in two categories.
The first is local spread. Infection can push beyond the tonsil into surrounding tissue, forming a peritonsillar abscess, a painful pocket of pus next to the tonsil that makes swallowing and even opening the mouth difficult. It can also spread to nearby structures, causing ear infections or sinus infections.
The second category is more serious and involves the immune system overreacting to the strep infection. Acute rheumatic fever can develop two to three weeks after the initial throat infection, even after the sore throat itself has resolved. The immune system, primed to attack strep bacteria, mistakenly targets the body’s own tissues, particularly the heart valves. Globally, there are roughly 470,000 new cases of rheumatic fever each year, though rates in the United States and other high-resource countries have dropped to fewer than 2 cases per 100,000 school-aged children. In some Indigenous populations in Australia, rates are dramatically higher, reaching 153 to 380 per 100,000 children aged 5 to 14. This is the primary reason doctors take strep throat seriously and treat it with antibiotics: not just to shorten the sore throat, but to prevent these downstream complications.
Recurrent Tonsillitis
Some people get tonsillitis repeatedly, and the reasons trace back to the same anatomy that makes tonsils useful. Deep, branching crypts can harbor bacteria in biofilms that antibiotics don’t fully clear. Each round of infection can also scar the tonsillar tissue, creating even more irregular surfaces where bacteria can persist. For people who experience multiple episodes per year over several years, tonsil removal becomes an option, not because the tonsils have failed, but because their architecture has become part of the problem.