Uh, for what type of infection? This is like asking for fuel for your car without knowing whether you need gasoline, diesel, compressed natural gas, electricity, etc. They're all reasonable fuel sources but work only in the appropriate and corresponding engine.
With that in mind, viral gastroenteritis is typically managed by supportive care. In other words, drink plenty of fluids. However, if you have bacterial, protozoal or parasitic diarrhea, then you need the appropriate antibiotic as determined by an evaluation of your stool. Unfortunately, there's not a good shotgun approach, although ciprofloxacin is a pretty good guess for the common traveler's diarrhea.
On the other hand, if your diarrhea is due to antibiotics, then you'll need something like oral metronidazole or vancomycin to combat Clostridium difficile. Probiotics might help, too.
But let's suppose you really mean the stomach as an organ, rather than just the vague abdominal area below your chest & above your pelvis. Helicobacter pylori is the infectious agent responsible for peptic ulcers. There are several 3-4 drug regimens that must be taken religiously several times daily for up to 2 weeks in order to insure adequate treatment and cure.
As you can see, there's a lot to consider. My suggestion is for you to talk with your family physician about your specific concerns. Good luck!
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