Just this year I realized how most of the "flues" I've "caught" were actually simple mold spore allergies. So I began tracking the mold spore in my area and noticed that the symptoms my kids would get also seemed to follow the same pattern (worse when mold spore is high, better when mold spore is low). Mold spore was high in this area earlier this week, and  two of my kids were sick (we had to take Ginny to the emergency room yesterday)

Today they're fine, and the mold spore is moderate.

You might try to see if there's a connection in your case. You can check the mold spore for you area of the country (sorry I have no link for those outside the US) at http://www.allernet.com/DAILY/mold.html