Have you checked for mold in your house?

My SIL and her kids were getting perpetually sick for months and months and it seemed to cycle, especially during the winter months for a few years straight. Something just wasn't normal. She finally had her house checked for mold (it was older, built in the early 80's) and yep....came back it was evident and was making them sick. It was extremely expensive to fix, but ended up being worth it as they don't get sick nearly as often now.

If your house is relatively newer the chances of that are pretty low, but still a possibility.