Straight from Harvard and I agree with it:
Don't smoke.
Eat a diet high in fruits and vegetables.
Exercise regularly.
Maintain a healthy weight.
If you drink alcohol, drink only in moderation.
Get adequate sleep.
Take steps to avoid infection, such as washing your hands frequently and cooking meats thoroughly.
Just taking vitamins isn't going to do shit unless you're following through on the other points here.
13+ years ago I was 105 pounds heavier than I am now. A few on this site knew me back then. I hit a wall, got serious about my health, and follow most of the points above. I should eat more fruits and veggies, probably. Anyway, I rarely get sick now, and I'm in the best shape of my life at almost 50 years old. I have yearly blood draws / health screens through my company and my Pharmacist Daughter looks at the numbers and says "I rarely see numbers this good from anybody, much less someone of your age."
So follow the above and take it seriously, your health / immune system will improve dramatically and you'll feel better for it.
I know, right? This totally gets swept under the run and it pisses me off.