Last weekend had a bad cold, turned into a sinus infection. My age is catching up with my body, just last year I ran 7+ miles with swine flu, this year I worked out on the Friday feeling it coming on (did back and shoulders) Sat and Sunday too congested to w/o or run. Monday through Wednesday too exhausted from the SI to even get off the couch, barely got to the ENT. Tried to do an easy 3 mile run Thursday, was miserable during and after even though I felt ok before. Rested Friday and then had terrible sleep (prob from not working out). Saturday ran 6 miles in hail and wind, Sunday had a heavy hard chest/tricep day. Today I was going to do biceps and shoulders, but decided to give myself another rest day, especially seeing I didn't get my usual sleep due to the Giant game. Tomorrow I want to try and get back in the swing, maybe a 7 miler considering it'll be 50. Wednesday I'll lift and by then hopefully be back to normal.
Anyway, point here is that a lot of stuff is dependent. Age, Sleep, Condition, Routine, but most importantly, body chemistry...
Just gotta trust how you feel, and try and be mindful of others to not infect them.