Stopping the diarrhea with an over-the-counter (OTC) medication may not be the best thing for your body, since the diarrhea probably reflects your body's attempt to get rid of a troublesome bug. If you do feel it's necessary, however, Pepto-Bismol is probably the safest OTC antidiarrheal medicine. It also appears to have a mild antibacterial effect, useful against traveler's diarrhea, which is usually bacteria-related.
Again, you're probably better off going without antidiarrheal medication. If you absolutely need some relief, however, you can try one of these OTC medications. Imodium A-D slows down the motility, or movement, of the gut; Kaopectate absorbs fluid. Elderly patients should use these medications only with their doctor's approval, because decreased motility can be dangerous in an infection and can lead to bigger problems.