i have mild allergies and was asthmatic as a kid, so if i try to ride with some sort of respiratory ailment, i come home hacking and wheezing, and will generally be out of commission for the rest of the day. i'll go lift weights if i really feel the urge to be physically active, but it just isn't smart for me to ride.

i'll tough out anything else short of a high fever though. the endorphin rush is fantastic, but i have to make sure to eat about 25% more calories after the ride than i would otherwise.