Help! Identify this spider bite. (IMAGE included)

This is a picture of my Dad.  He was bit on the face by something while he was in Colorado.  Antibiotics have brought him back to normal, but the redness on his face is still subsiding.  Even though it has mostly passed, we'd like to know what it was that bit him.  In addition to the swelling you can see in the picture, he had pain in all of his joints, fatigue and nausciousness.  He didn't feel a bite, and there is no noticeable bite wound.