Hi - I take vitamin C every day to ward off colds, and it seems to be working.
Vitamin C
Vitamin C, also known as ascorbic acid and -ascorbic acid, is a vitamin found in various foods and sold as a dietary supplement. It is used to prevent and treat scurvy. Vitamin C is an essential nutrient involved in the repair of tissue and the enzymatic production of certain neurotransmitters. It is required for the functioning of several enzymes and is important for immune system function. It also functions as an antioxidant. 
Current evidence does not support its use for the prevention of the common cold. There is
convinced of its worth, Pauling took 3 grams of vitamin C every day to prevent colds. Excited by his own perceived results, he researched the clinical literature and published "Vitamin C and the Common Cold" in 1970. He began a long clinical collaboration with the British cancer surgeon Ewan Cameron in 1971 on the use of intravenous and oral vitamin C as cancer therapy for terminal patients. Cameron and Pauling wrote many technical papers and a popular book, "Cancer and Vitamin C", that discussed their observations. Pauling