Is tonsillitis a serious illness?

Not usually: Our first president (george washington) died from a complication related to surgery for tonsillitis but generally tonsillitis is not serious... Other than causing severe pain. Complications include a peritonsillar abscess and severe swelling which can block your breathing. More importantly the streptococcus organism can produce toxins which can cause heart and kidney damage.

Yes: Particularly if caused by strep and left untreated. The bacteria can effect your heart valve and kidneys. Get it seen and treated.