I'm probably going along the same lines as everyone else, but the real truth is you have a higher chance of getting sick if you train really hard, and don't get enough nutrients or sleep. If you train harder than ever, and get the correct amount of nutrients replaced, which would be more than ever, and enough rest, which would be more than usual, you should technically have a lower chance of getting sick.
The first month of my wrestling season was the toughest but I was always eating for energy, and slept more too. I didn't get sick at the beggining of the season because of this. When I started cutting weight, and tournaments kept me up late, I started getting sick. I couldn't get around that, but in mountain biking, you can. Train harder, and at the same time get energy replaced, and rest. Nothing beats intense training.