One swollen tonsil is possible, although this is not very common. Most of the time, tonsillitis affects both tonsils, whether the cause is a viral infection or a bacterial infection.
In some cases, the inflammation begins on just one tonsil and, over time, spreads to the other. Early on, this can make it seem like only one tonsil is inflamed.
It is also possible for both tonsils to be inflamed but show different signs. One tonsil may have pus, look more swollen, or appear redder than the other, which can create the impression of one swollen tonsil.
Another situation that can also be normal is feeling pain or irritation on only one side of the throat, or noticing that the discomfort switches sides over the course of a few days.
In any case, if you have a swollen tonsil, the most recommended step is to see your primary care provider or an ear, nose, and throat (ENT) specialist to identify the cause and guide the most appropriate treatment.