Like, um, the flu cycle. We need a chance to get away from one another. I'll bet the kids are just as likely to get sick during vacation week. They'll be going sledding or skating or hanging out with friends or staying up late. Most kids are usually pretty worn out at the end of the week.
Teachers do use the week to catch up.
And not just physically. There are always papers to correct and lessons to plan. You need a break periodically to keep pace. I'm the one who needs a break. I have a job, too, you know.