The short answer is, yes, both forms of gastroenteritis can be highly contagious. A person with a stomach virus is contagious from the onset of symptoms to a few days after they recover. A stomach virus can be spread by consuming food or drink contaminated with the virus or by having direct or indirect mouth contact with a contaminated surface or person. In the case of bacterial gastroenteritis or food poisoning, those caused by certain bacteria, viruses, or parasites are contagious, but not so if it is as a result of chemicals or toxins.