And those were just your ordinary everyday illnesses! Now imagine how widely a virus that's two to five times as transmissible as flu would circulate through a dorm environment. It ain't pretty. And while those infected dorm residents themselves might be young and healthy enough to not have the infection be life-threatening, they could then carry it out of the dorms to all the other non-students they interact with every day, lots of whom aren't so demographically fortunate.
Closing the dorms is the right thing to do.