HBS has an article about people coming to work sick, the decline in productivity that results, and how to get them to stay home.
Whatever the shortcomings of current measurement tools and research, most people agree that presenteeism represents a problem for employers: When people don't feel good, they simply don't do their best work.
It's one thing to show that there's a problem, though, and another to demonstrate that there's something you can do about it—and, if something can be done, that the benefits will justify the investment. A central aim of presenteeism research is to identify cost-effective measures a company can take to recover some, if not all, of the on-the-job productivity lost to employee illness.
It reminds me of a time several years ago when one of the top engineers on my project got sick. He came to work anyway, and out of the next 10 days every person on that hallway had missed at least two days of work. It would have been better if he had stayed home.