Why just on the plane?
Travelling puts you in places, trains, taxis, buses, rental cars, etc. you don't normally use. You stand in endless queues in close contact with other travellers. Travellers rarely will cancel their trip just because a case of the sniffles, non-refundable tickets et al.
Avoid touching your face anytime the day of travel. If you've got the sniffles wear a disposable face mask. Do the rest of us a favour.
Why is ice OK, when water isn't. OK it was made in the flight kitchen but it's just as likely to be straight out of the tap. My philosophy, drink beer - water makes you rust.
Don't use soap and water? Why? Lather up and wash your hands while you sing 'happy birthday' (to yourself). That's about the right length of time.
It's a myth that the HEPA filtres aboard aircraft are not changed/cleaned often enough. And they are fine enough to filtre out viruses. But it gets repeated often enough it must be true. Do a search here and you'll find lots of discussion on this subject.
You are exposed to the air other people are breathing in two or three rows in either direction. The air circulation is lateral so you are not exposed to 'everyone else'. So if you are sitting next to a sniffler look out, but that guy 10 rows back with the sneezing fit probably won't infect you.
And if you do catch a cold while travelling, it is likely you caught it in a queue and not on the plane.