A physician spoke on NPR this morning, describing "walking pneumonia" and considering it as the condition Hillary likely has. It's pneumonia that allows you to walk around, not bad enough for bed rest - yet.
I contracted it twice: once as a restaurant manager working 70 hours/week and once while going to grad school (studying medicine, LOL) and working full-time. I continued working and missed only three days work each time, but was extremely tired for a week. After that, I had a bit of a cough that lessened over another three weeks. She'll be OK.
With frequent traveling and speaking; and fighting media scrutiny and her election opponent, Hillary may be tired for some time. Her physicians are likely much better than mine were, though.