I had strep every year around the holidays as a kid. It's a common, and easily treatable infection, but extremely dangerous left alone. It can go from bad to really bad, quickly, especially when it travels to the heart.
Anyway, within the folds of the tonsils, streptococcus and other bacteria can survive, waiting for your immune system to become depressed, where it powers up again, making you ill.
Hence, you may think you have got rid of the infection, but it is just in stasis inside you.
This is one of the primary reasons that physicians recommend that you have your tonsils removed once you appear to have overly frequent repeats of strep throat.
Sadly, the removal of the tonsils often means that infections tend to end up further down the throat after having the tonsils removed, which being nearer to the lungs raises the risk of more serious illnesses later.
http://www.mayoclinic.org/diseases-conditions/rheumatic-feve...
https://www.nimh.nih.gov/health/publications/pandas/index.sh...
Symptoms from the above link:
Presence of obsessive-compulsive disorder and/or a tic disorder Pediatric onset of symptoms (age 3 years to puberty) Episodic course of symptom severity (see information below) Association with group A Beta-hemolytic streptococcal infection (a positive throat culture for strep or history of scarlet fever) Association with neurological abnormalities (physical hyperactivity, or unusual, jerky movements that are not in the child’s control) Very abrupt onset or worsening of symptoms