how long has she been here? if a recent move, then she is still getting used to the US. If she has been her for a while, she is getting the same junk that is getting passed around. I have several friends with the same problem - adopted or not.
Also, look into homeopathic remedies. i know Garlic is a natural anti bacterial/microbial - but what kid is going to eat that? do some research on web MD and curezone.com
Also, I have my son on a GREAT OMEGA & Anti oxidant, and he has only gotten sick ONE day in his life (and no antibiotics) , even though he is adopted as a preemie. He can be around sick kids all the time and not catch anything since I give him this omega (not even vitamins).
Let me know if you want more info...
an PS, my niece and nephew are both adopted from Korea!