As flu season approaches, people soon will wake up to a tricky calculation: Should you drag yourself into work feeling awful? Or can you get away with staying home to heal?

Staying in bed poses a risk of falling behind or being seen as a slacker. But showing up sick, and grossing out or infecting colleagues, can be worse, and a growing number of employers are setting policies to discourage it.

"People get really, really ticked off at co-workers spreading germs in the workplace. There's nothing worse than being Typhoid Mary," says Annie Stevens, a managing partner at ClearRock, a Boston leadership-development and career-transition consultant.

This year's outbreak of whooping cough, or pertussis—projected to be the worst in 53 years—brings a new, highly contagious risk to the workplace, according to the Centers for Disease Control and Prevention. Recent outbreaks of hantavirus and norovirus are also raising concerns about transmissible diseases.

While email, Skype and teleconferencing make it easier in some jobs to call in sick and still participate in critical meetings or decisions, a person may end up being drawn into doing so much work they fail to take time to heal.

And it isn't always easy to tell when you're too contagious for the cubicle or conference room. (Having a fever doesn't help decision-making, either.) The 24-hour rule pediatricians preach to parents—that a child with the flu should stay home from school or day care at least 24 hours after the fever and symptoms go away—usually holds true for adults too, says Loreen Herwaldt, a professor of epidemiology and hospital epidemiologist at the University of Iowa Hospitals and Clinics.

But people with the flu can infect others a day before symptoms set in, the CDC says. Bystanders can catch the flu from a person standing as far as six feet away, usually via respiratory droplets spread when the sufferer coughs, sneezes or talks. And people with pertussis are highly infectious when the illness often looks like little more than a common cold, in the first one or two weeks.

Calling in sick can be awkward. Evelyn Hamilton of Washington, D.C., says her job as a customer-service coordinator for students with disabilities at a university requires a lot of energy and an upbeat attitude. Otherwise, she says, her clients feel she is "not being sensitive to their needs." While Ms. Hamilton's boss is understanding when she occasionally calls in sick because she's exhausted, "I do struggle with how I phrase it so that I'm not lying," she says. "I don't want to say my mother is sick…if that's not the case." Instead, she is brief and straightforward: "I'm not feeling well enough to come in today."

1 in 7 women have called into work and lied about being sick. The number for men is 1 in 5. Getty Images

Beyond the Sniffles A survey reveals 9 of the weirdest excuses for missing work. Employee's dog was having a nervous breakdown

Employee's dead grandmother was being exhumed for a police investigation

Employee's toe was stuck in a faucet

Employee was upset after watching 'The Hunger Games'

Employee got sick from reading too much

Employee was suffering from a broken heart

Employee's hair turned orange while dyeing it at home

Employee said a bird bit her

Employee's sobriety tool wouldn't allow the car to start Source: A 2012 survey of 2,494 managers and 3.976 workers for CareerBuilder

Many supervisors say they appreciate and respect a simple statement that an employee is too ill to work. Giving too many graphic details, or trying too hard to sound sick with "a very artistic fake cough, or saying, 'Oh, I have such a headache I can hardly talk,'" can spark suspicions that an employee is lying, says Rosemary Haefner, vice president of human resources for CareerBuilder, a hiring-consulting firm in Chicago.

Just 85% of employees say they are always honest when they call in sick. And 1 in 7 women has lied about being ill, compared with 1 in 5 men, according to a 2011 survey of 5,250 working professionals by the career website theFIT, a unit of recruiting-software maker Bullhorn.

"A lot of employers are wrestling with the sick-time conundrum"—how to get people to work when they're healthy enough, and keep them away when they're not, says Carol Sladek, a partner at Aon Hewitt, in Lincolnshire, Ill., a human-resource consulting unit of Aon PLC of London.

Some employers have policies that encourage sick people to come to work, offering cash or gifts for perfect attendance. Sal DeConcilio of Staten Island, N.Y., says he sometimes worked shoulder-to-shoulder with sick co-workers on a former job as a mail carrier because their employer allowed exchanging unused sick days for additional years on their pensions. "They'd be coughing and sneezing, working next to you," Mr. DeConcilio says.

Many employers have stopped handing out rewards for showing up, in favor of allotting employees a specified number of paid days off for any purpose, Ms. Sladek says. Some 51% have such paid time-off banks, up from 42% in 2009, according to an annual survey by the Society for Human Resource Management, a professional group.

To guard against people coming to work during a potential flu outbreak, Ms. Sladek says, some employers have prepared blanket no-fault sick-day policies that would temporarily allow employees as many paid sick days as they need without subtracting any vacation days. The message: "If you have the flu, stay home. Just stay home, until we all get better."

More than two-thirds of all health-related productivity losses spring not from sick people missing work, but from employees with chronic, contagious or other ailments who show up and perform poorly, according to a study in the Journal of Occupational and Environmental Medicine.

Some employees "have this sense that if I'm breathing, I need to go in and impress my supervisors," says Laura Bedrossian, a senior account executive at Peppercomm, a New York marketing and communications company. She recently told an intern who wasn't feeling well to go home. "If you have that drive to do something, you can ignore symptoms. But that's often the worst thing you can do," she says. Young employees, in particular, sometimes don't "realize how quickly you can bring an entire office down," says Ms. Bedrossian, who helps supervise interns in her office of 70 people.

Ms. Bedrossian sometimes uses work-at-home days allotted by her employer to keep going when she's afraid she might be contagious. She draws the line when her productivity falls too far, she says. If it takes her twice as long as usual to do a routine task, such as writing an email, "it's a good sign that I should take a sick day."

Doing career-damage control is better than playing the martyr and dragging yourself into work coughing and wheezing, Ms. Stevens, the career consultant, says. To allay fear of being labeled a slacker, touch base with co-workers via work-related emails, if necessary, she says. If you're missing an important meeting, ask to participate via teleconference if possible.

Colds travel fast in the open-plan office where Fred Yantz, of Hackettstown, N.J., works. If a co-worker looks especially ill, the magazine print-production manager says he drops a broad hint: "You're not looking so good. What kind of flowers do you want at your funeral?" His colleagues don't go home, but they often ask for an aspirin or Advil, which he keeps in a desk drawer. "It's not going to cure anything, but if it makes them feel better, that's fine," he says.

Write to Sue Shellenbarger at sue.shellenbarger@wsj.com

Corrections & Amplifications

An earlier version of the chart on the weirdest excuses for missing work incorrectly said there were 10 excuses when there are nine excuses.