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One of the peculiar things about tonsil infections is that they sometimes affect just one tonsil. When there is persistent swelling and tenderness on just one side of the throat, the first possibility that comes to mind is tonsilitis.

Tonsillitis is an infection most commonly caused by an especially aggressive bacterium known as Streptococcus pyogenes. This bacterium hides itself from your immune system by breaking down your red blood cells and using proteins from them to build a shell. Your immune system detects red blood cells and leaves the bacteria free to multiply.

Inside this purple or black blood clot, the strep bacteria begin to expand. Your tonsil begins to swell. It can become so large you have trouble breathing. You may have to breathe through your mouth, rather than through your nose. This exposes the lining of your mouth and your gums to air, and dries them out. Tiny cracks and crevices form, harboring bad breath (sometimes really bad breath) bacteria. You may start speaking with a "hot potato voice," and if the infection continues, you may develop trismus, or "locked" jaws.

As hard as it may be to believe in the twenty-first century, tonsillitis was once a common cause of death in children and teenagers. Then infection could spread to the lymph glands and muscles of the neck, causing the same kind of pain and swelling there it causes at the back of the throat. It could escape into the bloodstream and cause sepsis, which was inevitably fatal in the pre-antibiotic era. Even now, tonsillitis is something you definitely want to get treated to avoid long-term problems.

While tonsillitis caused by a strep infection is the most likely cause of swelling on just one side of the throat, it's not the only one. Here are some other possibilities:
  • Mononucleosis (also known as "mono" or kissing fever) can cause one-sided swelling.
  • Epstein-Barr virus, also associated with chronic fatigue syndrome after it has been in the body for a long time, can cause one-sided swelling. The virus does not cause the visible blood clots that identify strep.
  • Gastroesophageal reflux disease, also known as acid reflux, can cause irritation on just one side of the neck if the person who has it tends to get it at night while sleeping on that side. However, it is much more likely to cause problems across both sides of the throat, and it will not cause swollen lymph glands.
  • Tumors start on one side of the throat. They usually don't make blood clots, however.
  • Squamous cell carcinoma, generally thought of us as a "skin" cancer, can also pop up in the throat. It tends to spread through the branches of the tonsil into the lymphatic system. Squamous cell carcinoma of the mouth or throat is not a common condition. There are fewer than 8,000 cases a year in the United States. However, it is deadly if it is not treated.
  • Herpes infections can lodge in the tonsils. It would be unusual to have herpes in just one tonsil, at least for very long.
  • Leukemia can cause swelling, more typically of both tonsils, but occasionally of just one.

The more serious problems that are associated with swelling on just one side of the next are far less common, but treatment is more urgent. It's a good idea to see a doctor to get checked out. Chances are it's a strep infection of the throat, which is bad enough, but which is at least treatable. Getting a doctor to diagnose exactly what the problem is can help you feel better faster. It can keep you from giving your infection to other people. In very rare cases, it may save your life.

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