Step 1: Judge your sickness.

There actually are hard and fast rules for when you shouldn’t hit the skies—at least according to the Centers for Disease Control and Prevention (CDC).

Generally, you should bag a trip if you have a fever higher than 100.6 degrees Fahrenheit and symptoms such as chills; sweating; body aches and nausea; or vomiting and/or diarrhea, says Daniel Vigil, M.D., a health sciences associate clinical professor at UCLA’s David Geffen School of Medicine. It’s also not a good idea to fly if you have a contagious illness, including a respiratory disease or infection.

Why? Pressure changes make sinus and ear infections incredibly painful, and dry cabin air can further irritate upper respiratory conditions. The CDC also warns against travel if you’ve recently had surgery, a heart attack, or a stroke—these can increase your risk of blood clots and heart-related issues.

Then there’s the risk of contamination. Being within just two rows of someone sick can increase your risk of coming down with whatever they have, some research finds. The longer the flight, the more likely it is that you’ll infect others. Any of these instances, and you have plausible reason to plead your case.

Step 2: Call off the trip.

If you fit the Rx for a sick passenger, cancel the flight before the original scheduled departure time, says Brett Snyder, president of Cranky Concierge air travel assistance. If you don’t cancel in time, you lose the ability to use that credit toward future travel. Hit by the last-minute bug and unable to cancel 24 hours in advance? Don't give up hope just yet.

Step 3: Dig up the fare rules.

They’re outlined somewhere in the booking process, and describe exactly what is (and isn’t) allowed when it comes to cancelling. Print ‘em out right when you book, particularly if you bought a ticket via a temporary sale (fare rules can be tough to dig up later on). These will come particularly handy if an airline tells you it can’t refund your fare or waive a fee, but the fare rules state differently.