Tonsillitis is usually caused by a virus. Bacteria can also cause tonsillitis. The most common bacterial cause of tonsillitis is group A beta-hemolytic streptococcus (GABHS), which also causes strep throat.
How tonsillitis is spread
Tonsillitis is spread by close contact with an infected person. Droplets of disease-causing agents (pathogens) pass through the air when an infected person breathes, coughs, or sneezes. You may then become infected after you breathe in these droplets. Infection can also occur if pathogens get on your skin or on objects that come in contact with your mouth, nose, eyes, or other mucous membranes. Symptoms usually appear about 2 to 5 days after exposure.
A person with tonsillitis caused by strep bacteria is contagious early on and, without treatment, can remain so for up to 2 weeks. Antibiotics shorten the contagious period, and an infected person is no longer contagious about 24 to 48 hours after beginning antibiotic therapy.