If you look too sick to fly, airlines are supposed to stop you from boarding. For your own sake, you simply should not fly when you’re sick. Period. Not only could you make others sick around you, you could also make your own situation worse. If you have a bad cold, flu or sinus congestion, it will make it difficult for your ears to adjust to air pressure changes in the cabin. Such air pressure changes are hard enough on your ears when you’re well, let alone when you’re sick. This can make flying an uncomfortable or even painful experience.
You could rupture your eardrum if you fly when you’re sick and congested, which could lead not only to hearing problems but also balance problems. Is it worth the risk to fly and injure your ears when you’re sick, and then suffer the sensation of dizziness for months after the flight? Your ears are not as resilient as you may think they are.
Also, if you have a cold or upset stomach, you’ll probably want to make quite a few trips to the bathroom. This will eventually annoy other passengers if you don’t have an aisle seat, and on long haul or overnight flights in coach, you may not be able to get out of your seat at all if everyone around you falls asleep. Then you’ll have to choose between suffering in silence or constantly waking your fellow passengers. Either way, you will not be popular.
Yes, there may be additional charges to change your flight because you’re sick, but it’s just not worth the potentially serious consequences to fly while ill. The safety and welfare of you and the passengers around you depend upon you making the right choice. Let yourself heal while you’re sick and stay on the ground, for your own sake and for others.