I have never had a flu shot, for example. I note that this practice has not made me vulnerable to the flu. Interesting.

No, meaningless. Even during the great Spanish Flu pandemic (when there was no such thing as flu vaccine), most people weren't infected. In fact, during the many years before flu vaccine was invented, most people didn't get the flu. Just because you don't get it doesn't mean you aren't vulnerable. It just means you didn't get it.