How effective would wearing a face mask be in preventing you from catching a stomach bug from someone else?

A stomach virus/"bug" is not spread by respiratory or droplets. The most common ways to catch the stomach flu are by touching contaminated surfaces or objects and having skin-to-skin or hand-to-hand contact with an infected person, although contaminated food and water may also be a source of illness. So wearing a mask is not effective in this instance.

Usually spread of viral gastroenteritis is not airborne spread, but from hand to hand contact instead, so I may ask May not be very useful in the setting. Washing hands frequently winning contact with someone with a G.I. bug is a great idea. Good luck.

Do not expect a facial mask to protect you from a gastrointestinal disorder, as these are infectious agents entering via oral pathways and enter from hand contamination and/ or ingestion of the infective organism. Furthermore, based on the recent Cochrane analysis, it is not clear how effective masks are even with respiratory infections.