I've never done it from the flu, but I'll admit I've v*ed from the common cold. I was probably about 14, and all my life I hated the feeling of blowing my nose (because my ears would pop), so I would always just sniff up and swallow the mucus. Well, one night I guess I just swallowed too much of it on an empty stomach, and after feeling ill for several hours I got sick. I felt better immediately after and didn't have any more n* for the duration of the cold, and my doctor said that is a normal reaction if you swallow too much of your "secretions".

Moral of the story: if you have a cold, just keep blowing your nose and you'll be fine! Don't make the same mistake I did... Since that night I have never had any more problems with real n* during a cold.