Gastrointestinal parasites can kill frogs and they can be spread to other frogs by exposure to infected feces. A small gut load is usually not much of a problem, but with stress or other infections a small gut load can bloom and kill the frog. Most wild caught animals have some GI parasites and capture brings about stress, that is why most WC animals are in such rotten shape. The further away they came from, the worse shape they are usually in. Also most importers don't feed the animals that they sell. Often the frog's first meal after capture is the one it gets in the pet store.

As far as to what frog poop looks like, well it looks like poop.