Well, it's not as dramatic as I make it out to be, at least in this household. What drives me up the wall is when the trees and flowers (my home state is known for being flowery, natch...) here begin to send out pollen. Nothing like yellow powdery stuff all over my car and green loogie-like substances all over my bathroom sink each morning. But I digress.
The drama comes when I have to determine how much of my training is put to the side. I finished my base training - easy running, no hill work, no speed work, no racing - three weeks ago, and started doing some tempo work once or twice a week. If I'm smart (emphasis on the word "if") I only lose one week of training. I sensed the onset of the (upper respiratory) infection immediately on Monday afternoon and aggressively treated the symptoms.
If you fall ill, when do you adjust, and when do you drop, training as it's laid out? I've written on several posts about the "chin rule;" if the congestion is at the chin or higher (i.e., sinuses) an easier run will not do any harm. Sure, you're going to tick off people with that single-nostril blow - also known in some circles as a "Texas Hanky" - and they're certainly NOT going to want to slap you a high five after the run is completed. On the bright side, you won't have to worry about anyone borrowing your water bottle.
Once the congestion goes below the chin, most smart people say to forego the run. Inflammation of tissue are part and parcel of an infectious process. The difference between when the inflammation is above the chin and when it's below is almost akin to stuffing a rag in an older automobile's intake system; Shove a rag behind the air filter into one of two carburetors...inflamed sinuses might not be able to take in air, but you still have the mouth, which is almost as efficient. Jam that rag all the way down into the intake manifold and performance is truly compromised. Chest congestion also affects the small accessory (intercostal) muscles of the rib cage; normally those small muscles and the diaphragm have no problem pulling in that nice, fresh air. Inflame the airway passages into the lungs and you've got a problem. Anyone else old enough to recall a fast-food restaurant's "triple thick" imlkshakes? Without a large-bore straw you were doomed to wait until that drink melted some.
But lack of oxygen isn't the only good reason to consider taking a "sick day." Weidner, et. al. (1997) wanted to see if persons who had upper respiratory illness symptoms (e.g., fever, congestion, sinus drainage, muscle aches) ran differently than persons who were healthy. They found the ill runners had not only a increased stride length but also a decreased turn-over, most often associated with the symptom of a fever. The ankle joints were also seen to have a greater degree of extension while ill. Most running enthusiasts know a stride which is too long means the runner is more likely to have a gait with a heel strike. Add to this the decreased cadence - much like placing pressure on the shock absorbers of your car for an extended period of time - and there's a greater chance for fatigue and risk of damage.
Even more interesting, once the ill runners were well their running gait was not much different than a control group of physically-well runners. So, even though you feel compelled to run through an illness, the risk of injury outweighs the relative benefits of staying off your feet for a couple of days.