Yes I have heard that as well. Being exposed to certain bacteria will make you more immune to it, that is if it doesn't make you sick first. There is a chance that your boogers can contain an organism that can give you a viral or bacterial infection, but also ones that can be recognised by the body, and create antibodies against the germ without becoming sick. Same goes for any of the dirty things we touch, kissing, etc.

I Do It :)and im very healthy