Tonsillitis happens when your tonsils, two small masses of tissue at the back of your throat, become infected by a virus or bacterium. The germs enter through your mouth or nose, your tonsils trap them as part of their normal immune function, and the resulting fight between your white blood cells and the invading pathogen causes the swelling, redness, and pain you feel. Up to 70% of tonsillitis cases are caused by viruses, while bacteria (most commonly Group A Streptococcus) account for the rest.
How Your Tonsils Catch an Infection
Your tonsils sit at the entrance to your throat and act like filters. They’re packed with white blood cells whose job is to intercept germs before those germs can spread deeper into your body. Most of the time, your tonsils handle this without any noticeable symptoms. But when the number of invading pathogens overwhelms the local immune response, the tonsil tissue itself becomes inflamed and infected. That’s tonsillitis.
Healthy tonsils are pinkish and barely noticeable. Infected tonsils turn red, swell significantly, and sometimes develop white or yellow patches of pus on their surface. The surrounding lymph nodes in your neck often swell too, because they’re part of the same immune network trying to contain the infection.
Viruses vs. Bacteria
The viruses that cause tonsillitis are mostly the same ones behind the common cold and the flu. These viral infections make up the large majority of cases and typically come with other cold-like symptoms: a runny nose, cough, watery eyes, or diarrhea. Viral tonsillitis resolves on its own without antibiotics.
Bacterial tonsillitis, on the other hand, tends to look different. Group A Streptococcus is the main culprit, and infections caused by it are commonly called strep throat. Strep typically does not come with a runny nose, cough, or red eyes. Instead, the hallmarks are a high fever, swollen lymph nodes in the neck, and white patches on the tonsils without the usual cold symptoms. This distinction matters because bacterial tonsillitis does require antibiotics to clear the infection and prevent complications.
How It Spreads From Person to Person
Both viral and bacterial tonsillitis spread through close contact with an infected person. The main routes are respiratory droplets (from coughing, sneezing, or talking) and direct contact with contaminated surfaces like shared cups, utensils, or doorknobs. When you touch a contaminated surface and then touch your mouth, nose, or eyes, the pathogen can reach your tonsils.
This is why outbreaks cluster in places where people are in close quarters. Schools, daycare centers, and college dorms are common hotspots. Winter months see the highest rates of tonsillitis because people spend more time indoors in close proximity, and the viruses responsible for most cases thrive in cooler, drier air.
If you or your child has been diagnosed with strep throat, the CDC recommends staying home from work, school, or daycare until the fever is gone and at least 12 to 24 hours have passed since starting antibiotics.
Who Gets It Most Often
Children and adolescents are far more susceptible to tonsillitis than adults. Their immune systems are still developing, and they encounter new pathogens constantly in school and group settings. Tonsils are also proportionally larger and more active in children, which means they’re trapping more germs and have more opportunity to become overwhelmed.
Environmental irritants also play a role. Exposure to cigarette smoke or heavy air pollution can irritate the throat lining and make the tonsils more vulnerable to infection. Keeping children away from secondhand smoke reduces their risk.
Why Some People Get It Over and Over
Some people seem to get tonsillitis multiple times a year, and researchers at Washington University School of Medicine have found a likely explanation. Bacteria can form organized colonies called biofilms in the warm, moist folds (called crypts) of the tonsils. These biofilms act as reservoirs of infection that persist even after a course of antibiotics.
The reason biofilms are so stubborn is their structure. After bacteria attach to the tonsil surface, they secrete a sugar-like material that forms a protective shell around the colony. This matrix shields the bacteria from both antibiotics and the immune system. Research has shown that bacteria living inside a biofilm can be up to 5,000 times more resistant to antibiotics than the same bacteria floating freely. To make things harder, six to nine different species of bacteria can coexist in a single tonsillar biofilm, which means no single antibiotic may be able to eliminate all of them.
This is one of the primary reasons doctors recommend tonsil removal (tonsillectomy) for people who experience recurrent infections. Removing the tonsils eliminates the physical surface where biofilms form.
How Doctors Tell Viral From Bacterial
Because viral tonsillitis doesn’t need antibiotics and bacterial tonsillitis does, figuring out which type you have is a key step. Doctors use a scoring system that considers five factors: your age, whether you have swollen lymph nodes in your neck, whether you have a cough (a cough points toward a virus), whether you have a fever, and whether there’s visible pus on your tonsils. The combination of these factors produces a score that determines whether a strep test is warranted.
If the score suggests a possible bacterial cause, you’ll get a rapid strep test, which involves a quick throat swab and produces results in minutes. For children, a negative rapid test is often followed up with a throat culture that takes a day or two to confirm the result, because missing strep in kids carries a higher risk of complications. For teens and adults, the rapid test alone is usually sufficient.
What Happens During Treatment
Viral tonsillitis is managed with rest, fluids, and over-the-counter pain relievers to bring down fever and ease throat pain. Most cases improve within a week or so.
Bacterial tonsillitis requires a full course of antibiotics, typically lasting 10 days. Starting antibiotics shortens the duration of symptoms, reduces the chance of spreading the infection to others, and lowers the risk of complications. Even if you feel better after a few days, finishing the entire course is important to fully clear the bacteria and prevent the kind of resistant biofilms that lead to recurrent infections.
Complications of Untreated Bacterial Tonsillitis
Viral tonsillitis rarely causes serious complications. Bacterial tonsillitis, however, can lead to significant problems if left untreated. The most common serious complication is a peritonsillar abscess, a pocket of pus that forms next to the tonsil. An abscess can cause severe one-sided throat pain, difficulty swallowing, and a muffled voice.
If an abscess isn’t treated, the infection can spread into the neck and chest, potentially blocking the airway. The abscess can also rupture, sending infected material into the lungs and causing pneumonia. In rare cases, untreated infections can lead to sepsis (infection spreading to the bloodstream) or inflammation around the heart. These are medical emergencies, but they’re preventable. Getting tested and treated for strep throat when symptoms point toward a bacterial cause is the straightforward way to avoid them.