Is there such a thing as complete immunity to the flu? I've only had it once, as a child, and never since.

No, you can't be completely immune to the flu. The influenza virus can be prevented by vacinations. Do you get vacinated regularlly? Maybe that's why you don't get it. Also, you could have a strong immune system. You can build a strong immune system by eating right. I've only had the flu twice.