Olive leaf extract, sage oil, myrrh, red algae, and freeze dried mushroom powder (mushroom harvest brand; I get the blend). I swear 1 TSP a day of that stuff in my MRP shakes has helped fend off illness for a long time, especially during times of poor quality sleep, when I tend to get sick.
I also really liked Rez-V. I think it helps.
I would cook with coconut oil if I were you, for the lauric acid.
Staying healthy isn't about just one variable; make some substitutions. Have a regimen. You won't NEVER get sick, but you'll see a difference in frequency of illness and duration. People forget how stats work: you will see TRENDS, not absolutes (e.g. never vs. always sick).