The immune system is strongly influenced by the intake of nutrients. For a long time there has been a controversy whether vitamin C can contribute to the prevention and therapy of the common cold. Several cells of the immune system can indeed accumulate vitamin C and need the vitamin to perform their task, especially phagocytes and t-cells. Thus a vitamin C deficiency results in a reduced resistance against certain pathogens whilst a higher supply enhances several immune system parameters. With regard to the common cold different studies including meta-analyses underline 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.