Hillary! and her staff are probably searching medical books looking for some condition that could be an excuse for delay and to gain sympathy.
The problem with using this tactic is that it will cause people to once again question her health and whether or not she's physically capable of handling the presidency. I think a more likely scenario will be a sudden health crisis for Bill, causing Hillary to to delay her appearance before the select committee while she "keeps a vigil by Bill's side during this very difficult time for the Clinton family."
I would not want Trey Gowdy questioning me. It should be interesting to say the least.