When a miserable cold struck Kim Colabella in early December, duty called. Her supervisor and several colleagues were out of the office, and Colabella determined that, ailing or not, she needed to keep things going. So she took a cold pill, packed up her tissues and soldiered on to work.

But when Colabella arrived at Corporate Wellness Inc., a Mount Kisco, N.Y., company that coordinates employee health services for other companies, her sniffling, red-eyed arrival won her a chilly reception. Fellow employees reared back in horror when she came near and finally banished her to her cubicle.

The change in attitude: the recent shortage of flu vaccine.

Most healthy adults — more than 95 percent, by the federal government’s latest reckoning — are entering the flu season unvaccinated.

As a result, “presenteeism” — the practice of showing up to work sick — is now on the agenda of human-resources departments throughout the country.

As cold and flu season begins to take hold, companies across the country are issuing memos and posting signs in workplace restrooms, urging workers to wash their hands frequently, cover their coughs and sneezes, get enough rest and eat nutritious foods. Usually appearing at the bottom of this stay-well litany is an admonishment that few bosses have ever issued before, and many — even now — issue through gritted teeth: If you’re sick, stay home, employees are being told. And don’t come back until you’re better.

On average, says Dr. Kristin Nichol of the Minneapolis Veterans Affairs Medical Center, a normally healthy working adult who contracts influenza will be sick — and highly contagious — for five to six days. And he or she will miss, on average, one to three days of work.

Presenteeism costs companies as much as $150 billion in lost productivity, higher health-care expenses and cascading absences due to contagion, according to a recent report in the Harvard Business Review.