Too ill to work? Many people, and especially those with paid sick leave, stay home.

However, even people with sick leave regularly engage in "presenteeism" — going to work while ill. It's the opposite of absenteeism, and its effect on the workplace has been a topic long sequestered in academic journals.

As we settle into winter's cold and flu season, presenteeism is gaining interest in the American workplace for good reason: More costly than absenteeism, it is detrimental to employees and employers alike.

The U.S. Bureau of Labor says 39 percent of American workers — or 41 million people — do not have paid sick leave. That means a lot of people are showing up for work while under the weather.

In September, President Obama signed an executive order forcing companies holding federal contracts to provide paid sick-leave benefits to their employees.

On the face of it, being sick at work might sound like something employers might favor, with some work preferable to none at all. Besides, such employees display a strong work ethic, job dedication and loyalty. But research generally finds present-but-ill employees bring higher medical costs and greater reductions in productivity than absenteeism would cause.

A Society for Human Resource Management online article said presenteeism costs are "higher than the combined costs of medical care, prescription drugs and absenteeism," with estimated annual costs of $150 billion to $250 billion a year. That represents 60 percent of all productivity losses.

"Unhealthy workers are unproductive workers — and they're expensive," said Scott Wallace, a distinguished fellow at the Geisel School of Medicine at Dartmouth University. And the cost of poor health, he said, can be three to 10 times the total cost of all employee benefits.

"People around them get sick, and it increases stress and gets into a death spiral for employers," he said. "I'm mystified that employers can't figure this out in 11 seconds."

Employees who recover at home are more productive than persistently ill employees struggling at work to meet job demands, research shows.

People earning higher wages generally exhibit less absenteeism. People facing financial difficulties generally were more likely to show up for work when sick.

Ill employees make more mistakes and communicate less effectively and produce lower quality work. Presenteeism among pharmacists, one study found, resulted in more prescription errors. Downsizing actually increases absenteeism. In a real twist, research shows a higher propensity for medical workers to be on the job, even with contagious illnesses.