And, wear a mask.

I'd worry more about my own health than about annoying others. Flying with a sinus infection can be painful, and it can make the infection worse. I did it once and thought my head was going to explode from the pressure. Then it took a day for my ears to finally pop. On the flight home I took a decongestant and wore those air plane ear things--both helped. But if you are just dealing with a cough, then take a cough suppressant and bring lots of cough drops. That's the best you can do.

I once had to fly back home with my 3.5 yo when he had really bad croup. Some person on the plane asked if he had CF, which I thought was weird. But no problems with security, etc.