Tummy bugs are generally viruses but not flu; I do not know if elderberry does the same method on other viruses. I make elderberry syrup and use in flu season for prevention (1 spoonful a day) and if I feel any respiratory/flu like symptoms up the dose to 2-3X a day. For stomach viruses (not food poisoning!) elderberry may not be as effective as for flu, but it certainly cannot hurt. Elderberry is considered helpful in arthritis (anti-inflammatory probably why), and also anemia.
Thanks!