I find that the consistant commute keeps me healthier. Even when I feel like I'm coming down with something, it never hits me quite as hard. My g/f gets a few colds per season (allergy/sinus related stuff that develops into something else.) My uber-immune system just squashes stuff like that before it can take hold, because I haven't caught any of the colds that she's had.

If I do get sick, I will take a day or two off to let my body recover and use the energy to stomp down my illness before getting back on the bike. A stressed body is an unhealthy body.