I think it's from their change in diet. I know when my parents were no longer in control of my food purchases, i went wild and ate granola bars everyday for two months. LOL! I didn't get sick from it, but if their parents were strict controllers of their diets and did not let them eat a lot of junky food, a radical change in diet could have made them sick. Judging by the short duration of the illnesses, I am guessing it is not sv. It was probably just something they ate.