A recent study looked at the effects of elderberry extract (Sambucus nigra L.) on the H1N1 virus. They found that the extract prevents the H1N1 virus from infecting host cells (i.e., you), by binding to particular sites of the virus (called virions) that block prevent the virus from either recognizing your cells or from getting in. The researchers noted that the strength of the extract compared favorably to the known anti-influenza activities of Oseltamivir (Tamiflu) and Amantadine (both are anti-viral drugs). So keep in mind that this can be a safe and effective alternative for prevention. Once you get the virus, it can help slow it down, but doesn’t kill it. Other herbs can do that.
 
Phytochemistry. 2009 Jul;70(10):1255-61. Epub 2009 Aug 12.