Tonsillitis is a disease. The tonsils in the mouth get inflamed, and grow red. This may cause a sore throat or a fever. Usually the inflammation goes away on its own, but if it does not the tonsils may have to be surgically removed. This operation is called a tonsillectomy.
The disease is caused by certain viruses most of the time, but can also be caused by bacteria. It easily spreads between humans. The most important thing is to treat sore throat (pharyngitis) if it is caused by bacteria called streptococcus, with antibiotic for 10 days.