I do 21 miles a day, 5 days a week, year-round. I'm always running right on the edge. If something knocks my health down a peg, I really start to feel it. Last Thursday, I gave blood, and my ride home from the donor center and my commute the next day I felt like I was riding through molasses. I actually checked my bike twice to make sure the brakes weren't dragging. It was just me.
Same thing happens if I start to catch a cold. Colds can't actually get much foothold on me, a ride on the bike takes it out pretty quickly, but for the day or so it's trying to scratch at me, I can get run down.