Would you board an airplane knowing you had the flu? Would you worry about possibly infecting other passengers or airline crew?

An online survey sponsored by the National Foundation for Infectious Diseases found that two-thirds of Americans would fly knowing they had the flu. Close to half (45 percent) of those surveyed admit they would board with flu symptoms if they were going on vacation. Some 40 percent of business travelers admit they would fly with the flu. Those are the people who fessed up but the actual numbers may even be greater.

The decision whether or not to fly when you're sick is complicated because it's influenced by a number of factors:

Restrictive policies discourage medical waivers.

"With some airlines, it's the policies themselves that motivate people to fly when they're sick," says Andrew Schrage, co-founder of MoneyCrashers.com .

"U.S. Airways offers no medical waivers to assist passengers who are too sick to fly. Some like Delta Air Lines, on the other hand, treat the matter on an individual basis," he adds.

"Even when they do exist, airlines typically require proof of hospitalization to have change fees waived," says Alicia Jao, vice-president of travel media for NerdWallet.com .

When children fall ill, these waivers aren't likely to apply to their accompanying parents or siblings.

Flight cancellations fees can be punishing.

Flight cancellation fees can be punishing. Moreover, given the scarcity of seats on some routes, a replacement flight may be expensive, hard to rebook, or both.

"This encourages many people to fly despite being ill," says Schrage of MoneyCrashers.com. "Southwest Airlines doesn't charge these [cancellation] fees, but Delta, American and United all charge $150 per ticket to cancel or rebook," he adds.

Airlines are reluctant to kick sick passengers off planes for financial reasons.

While carriers have the right to deny boarding to a sick passenger, they rarely exercise that option.

"There is little incentive for airline personnel to go the extra mile and deny boarding for a sick passenger as that would create an empty seat and lost revenue," says Jao, of NerdWallet.com

Symptoms of communicable diseases aren't always apparent.

Even if airlines attempted to prevent passengers from flying sick, it is tricky for airline personnel (and sometimes even medical personnel) to discern whether someone has symptoms of a common cold or a more serious respiratory disorder, such as pneumonia or influenza -- or even whether someone has chicken pox (which can look like acne).

Thus, when gate employees or flight attendants have suspicions, they may turn their heads because they really can't be sure. During a pandemic or public health crisis, there is usually a high state of vigilance, but attitudes towards flying sick generally slacken and become more relaxed afterwards.

"We heard about numerous cases of feverish-looking people being told to travel another day during the H1N1 pandemic of 2009," says Rick Seaney, CEO of FareCompare.com .

Since then, when was the last time you saw a fellow passenger confronted by airline personnel and asked to leave a plane for flying sick?

Passengers can be self-centered.

Unfortunately, travelers often aren't concerned about the welfare of other passengers around them.

"I think people are selfish. It is all about their schedule, their costs, and they forget their own misery or risks to those around them," says one fight attendant who didn't want me use her name.

Precautions travelers can take to protect themselves:

Given all the perverse incentives for flying sick, what can a traveler do to protect him/herself?

-- At least two weeks before you travel make sure your vaccinations (e.g. flu, pneumonia) are up-to-date.

-- Consider trip cancellation insurance but assess the cost and read the small print.

"Trip cancellation will reimburse travelers up to 100 percent of trip costs if they are ill. Cancellation due to sickness is always a covered reason, as long as travelers provide documentation from a doctor which states they were unfit for travel," says Jessica Bell, a spokesperson for Squaremouth.com .

"If a traveler is already ill and they know they will have to cancel their trip, it is too late to buy trip cancellation insurance," says Steve Dasseos, president of TripInsuranceStore.com . "It's like wanting to buy fire insurance when your house is on fire."

-- If you think you may have been exposed to a bug that will render you too sick to fly, check out specific airline waiver policies before you select a carrier, or consider purchasing an unrestricted, totally refundable ticket (These may be unaffordable but you can sometimes snag them using frequent flyer miles).

-- Reconsider your decision to fly. If you know you have a highly contagious illness like pneumonia or the flu, be sensitive to the possible effects on other passengers who may be young, aged, or immunocompromised. If that doesn't stop you, think about how ill and uncomfortable you may begin to feel during the flight.

-- If you think the person next to you looks too ill to fly, ask the flight attendant to change your seat. When seats are available, airline personnel will usually try to honor your request. If no other seats are available and you're stuck next to someone who seems sick, limit conversation and try to face away from the individual.

Irene S. Levine, PhD is an award-winning travel writer and member of the Society of American Travel Writers (SATW). You can follow her blog for travelers over 50 at More Time To Travel or on Twitter .