I noticed something on a rewatch. When the sheriff starts having issues, he looks like he's having a stroke or something, but otherwise, he looks normal. We are then shown the scene where he is being inspected and they are trying to figure out what went wrong. Except now, he is covered in blood and has been scalped. Looks like at some point in between the fly landing on his face and that couple of guests leaving him, and him being retrieved by the Jeffrey Wright and co., he had a run-in with the Man in Black (I think it's unlikely the techs did that to him as part of their inspection, given that we haven't seen any of the other hosts).
Maybe everyone else saw this and I'm just really unobservant.
Before: http://i.imgur.com/zVImZ3U.png
After: http://i.imgur.com/7XfiheH.png