Propylene glycol is an air germicide so, vaping is like the mouth, throat, nose, and lung equivalent of hand sanitizer. The second hand vape is probably sanitizing some of the outside air too. I used to get sick every time I came in contact with someone that was sick. Now , most of the time, I feel the beginning symptoms of a cold/flu and vape them away. That has never happened to me before. Even when I was a child before I smoked. The beginning symptoms always lead to full blown cold/flu but not since I started vaping. Also there is the not smoking thousands of carcinogens that helps boost my immune system. That's a huge part of it but I can definitely tell that I'm vaping away colds and the flu after I have caught them.