Anytime you get lots of people in close contact, you risk spreading everything from colds to the flu, to worse.
When I was in basic at Ft Polk, LA, one of the guys in my barracks was removed due to meningitis. I and my fellows thought nothing of it because we were young and invincible. He was gone for several weeks, returned, and graduated (if that is the correct term) from basic with the rest of us and then went on to regular Army duty. I never saw him again.