I don't think anything can put a bigger damper on progress, ambition or motivation than physical illness. As you may guess, I am ill.
I hate being sick, but it is worse when you have a laundry list of things to get done, deadlines and schedules to keep and people relying on you. The only way through it is to take doses of medication and continue. It is the frustration and reality that regardless of modern wonders, the common cold or flu that slows me down is the same one that told my ancestor to stay in his cave and go hunting another day.