Posted in Web Stomach flu: How long am I contagious? – Mayo Clinic January 24, 2021 You can be contagious from a few days up to two weeks or more, depending on which virus is causing your stomach flu (gastroenteritis). A number of viruses can cause gastroenteritis, including noroviruses and rotaviruses. https://www.mayoclinic.org/diseases-conditions/viral-gastroenteritis/expert-answers/stomach-flu/faq-20057899