The scientific knowledge supports the following in strengthening human immuno-response:
Zinc (e.g. zinc gluconate, don't overuse over long periods, will mess up your cell chrome levels)
Echinacea (fluid extracts diluted in alcohol absorb the fastest, don't overuse the alcohol part

)
Ascorbic acid aka Vitamin C (pref. time released. c. 500 mg will saturate blood serum)
Asetylic-salisylic Acid aka Aspirin non-buffered (half a tab will do, don't overuse, can cause stomach ulcer/intestinal bleeding if used over long periods of time)
Garlic (Allicin being the principal active agent, fresh garlic is the best bet)
Ginger (gingerols being active agents, esp good for viral outbreaks with associated joint pains/inflamations)
B-vitamin multi-formula (some B-vitamins needed for synergistic effect others have immuno strengthenig effects, namely B6, B12 and folic acid)
Also, regulating your non-diuretic fluid intake and keeping your body temperature non-regulated pharmatically (i.e. no ibuprofen or other fever reducing medicine, if you actually fall ill) will help you recover faster.
Non-diuretic means that it does not increase your fluid outtake (i.e. need to go to the bathroom). Alcohol, coffee, tea, beverages and many highly processed fluids can contain diuretic agents. Good bet is plain water or fruit/vegetable juices.
If you follow this formula and have a decent basic aerobic and cardiovascular condition, it is highly likely you will go for years or decades without catching a single flu that really brindgs you down with fever.
You will catch flus, but your body will adapt, repel and form a resistance to the strains extremely quick, without you having to go through a long period of sickness. You probably won't get a fever or have to be away from work or other activities.
Then again, sometimes it's good to fall sick for a while as it can do wonders to the psyche (after you've healed, that is)

regards,
Halcyon