The tissues won't do much. The vapor spray coming from them is still airborne. Instead of offering tissues, bring some Coiricidin or something. Plenty of people fly with colds. They're just smart enough to be on drugs, so it's not a huge deal. I should also point out, some people have sinusitis for reasons different than cold/flu and are not contagious. That said, you're exposed to nasties every time you fly whether someone is next to you or not. There's nowhere that you go any time that is sterile.