Few things are more important to me when travelling than my health - and I could completely relate to the OP as I too have changed seats and considered getting off a plane - or asking the flight crew to remove a clearly ill passenger (but have never actually done that). I do wear masks as well - although not often in the west as I only travel with a few when away from Japan... I'm not a germaphobe but do admit to cleaning things with wipes before eating / using certain things lol... An airplane is one of the "dirtiest" environments even when passengers are relatively healthy - adding in a clearly ill passenger really increases the risk of you getting sick - and that's something few of us that travel regularly want to deal with...