"Stomach Flu" isn't the flu at all. Some people call diarrhea, nausea, or vomiting that lasts for a day or two "stomach flu." This is inaccurate. That illness is caused by other viruses, and is very different from true Influenza.

In adults, Influenza rarely manifests itself in nausea, vomiting, and diarrhea, but sometimes does so in children.