When I was in first grade, I spent almost the entire school year battling recurring bouts of strep throat.
I don’t know how much effort anyone put into researching the cause of these continual infections (paging Dr. House); all I remember is going back to the doctor again and again to have my throat swabbed and get another round of pink, icky-sweet, chalky liquid antibiotic.
The source of the strep bacteria was revealed after school let out for the summer: When my teacher underwent some pre-surgery blood tests, she discovered that she was carrying the bacteria. No symptoms. No clues, except for one otherwise healthy little girl missing a lot of school — and not being altogether that unhappy about missing school.
I still attribute the ultimate demise of my tonsils to this epic battle with the strep bacteria. (And I don’t hold a grudge against Mrs. Buffington. I DO hold a grudge against the series of doctors who, over the next 14 years, refused to consider taking my tonsils out as they slowly rotted away.)
You obviously can’t launch a full-fledged medical investigation of everyone your child comes into contact with (although, again, Dr. House seems to get away with such antics quite frequently), but the link seems obvious now. I don’t even know that anyone could make the connection today — although I do imagine that such a discovery today would involve litigation.
No matter how careful you are, or how protective you are of your children and loved ones, there’s always one thing that you don’t see coming. Expect the unexpected. And insist that the tonsils come out NOW.
Entirely Adequate 
Something else to remember, is that just because you hear hoofbeats, it doesn’t necessarily mean horses. It could be zebras.
Although, in my case the search for zebras ended in the discovery of an out-of-place horse.