I have not been sick in three years, I am 40 years old, have a somewhat streesful job, and two young kids. Also, I don't use anti-bacterial soaps or constantly wash my hands like the media tells you. Finally, never had a flu shot. Here is what I do during the period from September thru March in order of importance.
1) Chew 1 raw crushed garlic clove a day
2) Exercise to a sweat at least 45 minutes once a day
3) 2000 mg of vit c, 30 mg of zinc, and 1000 IU of vit d daily
4) Greatly limit the intake of sugar and white flour. Both cause insulin spikes which greatly reduces the effectiveness of your immune system.