I swear by Sambucol (available over the counter at CVS nationwide). Once you're sick, you'll need a couple tablespoons every two hours at least, but a couple teaspoons twice a day is enough to ward off sickness from others.

I was dosing on Sambucol and 1500mg of Vitamin C all last week while traveling around sick people - all I got was a slight throat tickle while the virus tried to take hold, but with Sambucol I was able to fight it off. The elderberry extract has antiviral properties which have been subjected to laboratory review and found effective.