I've cycled 4-6 days/week on average for years, and I rarely ever catch colds or get the flu. It might just be that I have naturally high resistance, but I think that being in good shape helps. However, if you are really exercising hard -- to the point of overtraining or close to it -- I think it makes you more susceptible to illness. In fact, that is one of the signs that you are overtraining -- when you get sick a lot.