Profile everyone on those planes.
Alternate thought: Check the air filtration and recirculation systems on those planes, as well as anything else common to them.
I say this because I don't get sick very often in general, but there's a much higher probability of my catching something on a flight. I can't be alone--ever see the number of Chinese who wear surgical masks on airplanes?
If there was some flu epidemic in Europe going on...you would see evidence of this. So to have this many in some flight activity, it poses questions.
I might be looking at airport workers who recently returned from Mecca, and they are passing along some swift flu to passengers making their way through the airports.