AAQ, it’s something most parents teach their kids, whether they’re Jewish or not. It’s common sense, but it’s not something I was particularly worried about, even though I’m on the overcautious/, nervous side regarding health issues b’chlal. I was a very fidgety kid and my parents picked their battles. I’m not saying it’s the right thing to do, but I don’t think I got colds very often; it could be that my immune system is used to it due to my never having been careful about such exposure.
There’s an established link between gingivitis and heart disease. One cannot contract gingivitis from doorknobs. I don’t believe that even a frequent cold poses any serious issue.