“Can vitamin C prevent the flu?”

"There is controversy whether vitamin C can contribute to the prevention and therapy of the common cold. Several cells of the immune system do accumulate vitamin C and need the vitamin to perform their task. Thus a vitamin C deficiency results in a reduced resistance against certain pathogens whilst a higher supply will enhance several immune system parameters. With regard to the common cold, different studies show that the prophylactic intake of vitamin C may slightly reduce the duration of the illness in healthy persons, but does not affect its incidence and severity. Supplementation of vitamin C is most effective in cases of physical strain or insufficient intake of the vitamin. With regard to the therapy of the common cold the application of vitamin C alone is without clinical effects."

Having said that, I do believe that vitamin C obtained from natural resources via fresh fruits and vegetables will be more effective in boosting your immunity vs. taking "pills" or "herbal supplements." Also, a daily regimen of moderate exercise, and hydration all contribute to well-being.