Assuming it perforates the intestine - big infection from fecal matter getting into the abdominal cavity. It's called peritonitis, and AFAIK it's pretty much fatal if untreated. Severe abdominal pain as the infection spreads, all the vomiting and diarrhea cleans you out, and then the infection starts to shut your intestines and the rest of your organs fail while you go into sepsis and die. It's not fast, either. It can take several days.