I'll just offer my viewpoint here: I suspect damp mucus membranes and the phlegm/thick mucus that often is produced during illness are not the same things! Although I suspect sniffing that mucus back down is not directly harmful, I assume it then either comes back up/out and/or has to be eliminated using the body's own energy resources, e.g. one's kidneys, liver, etc., [something that during illness, is particularly limited], i.e. where practical, my own view would be that expelling it by "blowing one's nose" would be the better approach, overall health-wise, than sniffing it back down and swallowing.