I seem to be immune to flu in its various strains through the years. Didn't get it even when I discovered I had been up close and personal with people who were at peak shedding times. I don't know that for a fact, but it fits the data. Further, the one time I ever did get it was 7 years ago when I was dealing with an issue that lowered my immune system while it was going on. I didn't know there was such a thing, but now I've been told by people who should know that most viruses are going to get kicked back by a small percentage of the population. Guess I'm one of the flu ones.
I seem to catch Covid pretty easily though. Fortunately, the short term affects are not that bad for me, and that one bout I had with the flu was way worse.