It's been 10 years since the flu season was this bad, according to the Centers for Disease Control, and it's going to be costly as well as bad.

HRDive reports that figures from Challenger, Gray and Christmas peg the dollar sign at $9 billion in lost productivity. Eleven million employed adults are expected to come down with the flu this year, and the consulting firm's calculation assumes workers with the flu will miss four days of work.

The flu spreads quickly, and not only do employees staying home with it cost employers, but the ones who decide to "tough it out" and come to work cost, too. After all, they're bringing their germs to work and spreading them generously among their coworkers. But the number of them who do so might surprise you; according to Staple Business Advantage's cold and flu survey, a whopping 80 percent of employees came to work ill last year.