Hm. Well, I wouldn't really know now, since I graduated and am currently bandless. ( crying ) But I get sick easily, especially during marching season. I think it would really have to do with what and how you got sick.
Most germs that get us sick are distributed through the air, or from sharing food or drink with a sick person. Another would be touching anything that a sick person has used. It also depends on the illness' ability to get people sick. If it's a cold, you're all likely to get it some time or another.
My theory is that when you spend enough time with sick people, you will eventually get sick. A cold is very common. (Heck, I get it at least once a year. XP ) It could be an outbreak, and you should all just take your medicine and lots of vitamin C. It can also be the weather. The year I had the worst cold ever was during an especially rainy marching season, and it rained just about every game. I was even told by a band member that I should stay home to get better.
Just let it runs its course and take care of yourselves. Since it is such a big group, it will not be hard to not catch anything at all. Kudos to you for not getting sick so often! But you'll all be okay. Remember: take your medicine, and have lots of vitamin C!! ^_^