I didn't realize it until reading this but apart from my seasonal allergies, since I've begun bike commuting, I've never been sick. Although when I was sick before commuting, I did find that if I was well enough to be mobile, a short bike ride made me feel a lot better. I think it just has to do with getting the juices flowing and breathing in fresh air, it's gotta be better for you than just sitting around in the stuffy indoors.