Tonsillitis starts when a virus or bacterium infects the two oval-shaped pads of tissue at the back of your throat. In 70% to 95% of cases, a virus is the trigger. The remaining cases are bacterial, most commonly caused by group A strep. The infection typically begins with a scratchy sensation in the throat that worsens over a day or two into full-blown pain and swelling.
What Happens Inside Your Throat
Your tonsils sit at the entrance to your airway like a pair of immune system guards. Their surface is covered in small pockets and folds that trap inhaled or swallowed germs. Normally, the tonsils sample these pathogens and help your body build immune responses against them. But when a virus or bacterium overwhelms that defense, the tonsil tissue itself becomes infected.
Once a pathogen takes hold, your immune system floods the area with white blood cells. That immune response is what causes the redness, swelling, and pain you feel. If bacteria are involved, dead cells and bacteria can collect on the tonsil surface as white or yellow patches. The lymph nodes in your neck, which filter fluid draining from the area, often swell and become tender at the same time.
Viruses vs. Bacteria
The vast majority of tonsillitis cases are viral. Common culprits include adenovirus, Epstein-Barr virus (the virus behind mono), herpes simplex virus, and measles virus. Viral tonsillitis tends to come alongside other cold-like symptoms: a runny nose, cough, and general fatigue.
Bacterial tonsillitis accounts for 5% to 15% of adult cases and 15% to 30% of cases in children ages five to fifteen. Group A strep is the most frequent bacterial cause. Strep-related tonsillitis looks different from the viral version: it usually hits harder and faster, with a high fever and no cough. You’re more likely to see distinct white patches on the tonsils and significant lymph node swelling in the neck. Doctors use a set of clinical signs called the Centor criteria to estimate the likelihood of strep. When three or four of these signs are present (tonsillar coating, swollen neck glands, fever above 38°C, and no cough), the chance of a strep infection ranges from roughly 32% to 56%.
How the Infection Spreads to You
The germs that cause tonsillitis travel through respiratory droplets. Someone nearby coughs or sneezes, and you inhale the droplets or touch a surface they’ve landed on and then touch your mouth or nose. Sharing drinks, utensils, or food with an infected person is another common route. Kissing can transmit it directly.
This is why tonsillitis is especially common in school-age children. They spend hours in close quarters, share supplies, and are less consistent about hand hygiene. The infection circulates easily in classrooms, daycare centers, and similar environments. Bacterial tonsillitis peaks during late fall and winter, when people spend more time indoors together.
The First Signs You’ll Notice
A sore throat is almost always the first symptom. It often starts as a mild scratchiness at the back of the throat, easy to dismiss as dryness or the beginning of a cold. Within a day or so, swallowing becomes noticeably painful, and the soreness deepens.
Other early signs include:
- Red, visibly swollen tonsils that you can see if you open wide in a mirror
- Fever, ranging from mild to high depending on whether the cause is viral or bacterial
- Tender lumps in your neck from swollen lymph nodes
- A muffled or throaty voice, sometimes described as sounding like you’re talking with a hot potato in your mouth
- Bad breath from bacterial buildup on the tonsils
- Headache, stomach pain, or stiff neck, particularly in children
Young children who can’t describe their symptoms may drool more than usual because swallowing hurts, refuse food, or become unusually fussy. These are often the earliest clues parents notice before a throat exam confirms the diagnosis.
Why Some People Get It Repeatedly
Children between five and fifteen are the most affected age group, partly because their immune systems are still learning to handle common pathogens and partly because of constant exposure in group settings. As adults, the tonsils naturally shrink and become less active, which is one reason tonsillitis becomes less frequent with age.
Some people develop recurrent tonsillitis, meaning multiple episodes per year. The tonsillar crypts (those small folds on the surface) can harbor bacteria between infections, creating a cycle of reinfection. Chronic inflammation can also make the tissue more vulnerable to new pathogens. When episodes happen frequently enough, typically seven in a single year, five per year for two years, or three per year for three years, removal of the tonsils becomes a consideration.
What Can Go Wrong If It’s Left Untreated
Most viral tonsillitis resolves on its own within a week to ten days. Bacterial tonsillitis also clears in many cases, but untreated strep carries risks that viral infections don’t.
The most common complication is a peritonsillar abscess, a pocket of pus that forms in the tissue next to the tonsil. Signs include severe one-sided throat pain, difficulty opening the mouth, drooling, and a visibly swollen area pushing the tonsil or uvula off-center. This needs medical drainage.
Rarer but more serious complications of untreated strep include rheumatic fever, which can damage the heart valves, and post-streptococcal kidney inflammation. These systemic complications are uncommon in developed countries because strep is usually identified and treated, but they remain a risk when bacterial tonsillitis goes unrecognized. A child or adult with neck swelling, worsening pain despite rest, difficulty breathing, or signs of spreading infection needs prompt medical evaluation.