Massive dosages of vitamin C. When we're sick, or stressed (and who isn't these days) we secrete adrenaline, and the more adrenaline we secrete, the more vitamin C we consume. The RDA for vitamin c is 60mg. Unfortunately, this is barely enough to prevent scurvy in many people.
Most people can take up to 15grams (15,000mg) of vitamin C without reaching "bowel tolerance," or the point where saturation of vitamin C is reached and the result is diarrhea. When someone is sick, the requirement of vitamin C goes up to about 30 grams to 60 grams per day (30,000mg to 60,000mg).
I take 8 grams per day and have yet to reach bowel tolerance. These dosage levels typically prevent colds and flus in most people. Vitamin C is an anti-viral, anti-histamine, anti-depressant and bacteriocidal among other things. There is a phenomenal book on the subject by Steve Hickey, PhD and Andrew Saul, PhD (a brilliant man) called Vitamin C: The Real Story. It's a fascinating read.