Is a bacterium that lives in our digestive systems, where it has a beneficial role; but it becomes dangerous when our food or water is contaminated with feces. sciemce

The answer would be: Escherichia coli

Escherichia coli is normal flora of the human gastrointestinal tract. Escherichia coli has a beneficial role because they can prevent infection of other harmful bacteria and produce some substance like vitamin K. But some strain of E. coli is dangerous and could cause bloody diarrhea, vomiting, and stomach ache.