A lot of people believe that Vitamin C helps build your body's resistance to flu and other viral illnesses. You can find lots of web sites that talk about using Vitamin C for this purpose. However, there is no medical evidence to show that there is actually any benefit to taking Vitamin C in terms of treating or preventing the flu.
Orange juice is good, but do not expect extra benefits because of vitamin C contained. Studies have shown that vitamin C does not prevent and it does not treat respiratory infections.
Better start taking supplements of zinc or ginseng. The body uses zinc (max. of 15 mg / day) to produce white cells that fight germs. Ginseng supplements strengthen the immune system.