I suppose that, like other diseases, for some people it lasts longer than for others. When I used to get flu, it was generally of much shorter duration than most everyone else.
I believe we all do have different immune systems. I am slow to get ill and I get over it quicker generally as well.
I also believe that exposure to a variety of low level germs as a child can help build up a strong resistance. I spent a little over ten years on a farm when I was three to about fourteen. I credit exposures to various things on that farm to my elevated immunity.
As a kid you don’t wash your hands as often as you should. I probably exposed myself unwittingly to various things that my body fought off and I didn’t even know it.