What's an all natural product that keeps immune system strong?

I’m looking for something not unhealthy at all, so you know, I’m looking for something natural that I can take for life I guess periodically, once a day maybe, or twice a week would be better lol.

Because I have been taking 2,000mg of Vitamin C for months now to stop myself from getting sick and yet I got sick 5 days ago with a cold!! For example I heard people who take “Elderberry” never get sick, that it really helps them? Maybe I should get that?

I am 19 and male thank you!