Now the one thing that I will fault the airlines with, is their sinks. You cannot wash your hands properly at them, and so that might be why it seems you are more likely to get sick after being on an international flight when someone else is sick, which has happened to me. Their sinks suck. There's no water pressure, and you have to hold the little lever down to have water come out, and so you have to touch something that has been touched by others to get water. I think maybe it would be prudent at that point to have anti-bacterial gel to put on your hands before you eat or touch your face. Indeed, maybe it would be a good idea for the airlines to hand out anti-bacterial towelettes when you board and supply them throughout the flight.
What I think this boils down to, is if you're sick, you should really ask the airline to change your ticket if it's possible, most of them are being really lenient right now about waiving change fees. Don't fly if you're sick, it's rude to the other passengers. And if you have to fly, maybe consider wearing a face mask so you keep your water droplets to yourself.