Vitamin D which improves immunity to flu by increasing production of antimicrobial agents in the body. These antimicrobials can fight against the influenza virus. It's no coincidence that the flu season coincides with the lowest annual levels of Vitamin D in the body.
Vitamin C not only boosts levels of infection fighting white blood cells, but it also increases the number of interferon antibodies which work to keep colds & flu from even entering cells.
Zinc is well-known for helping people prevent colds and the flu. Zinc strengthens the immune system and improve white blood cell count. Taking zinc helps to shorten the length of the illness.
Echinacea can help prevent the onset of colds and other respiratory illness, echinacea can also help to reduce duration of colds, flu, and sore throat.