Viral (e.g. rotavirus)
Bacterial
Campylobacter
Shigella
Salmonella (non-typhi)
Escherichia coli
E. coli 0157:H7
Yersinia enterocolitica
Vibrio cholerae
Clostridium difficile
Parasitic
Giardia lamblia
Cryptosporidium
Entamoeba histolytica
Cyclospora
Microsporidium
Toxin
Staphylococcus aureus
Bacillus cereus
Campylobacter
Shigella
Salmonella (non-typhi)
Escherichia coli
E. coli 0157:H7
Yersinia enterocolitica
Vibrio cholerae
Clostridium difficile
E. coli 0157:H7
Giardia lamblia
Cryptosporidium
Entamoeba histolytica
Cyclospora
Microsporidium
Staphylococcus aureus
Bacillus cereus
GI Bleed
Appendicitis
Mesenteric Ischemia
Diverticulitis
Adrenal Crisis
Thyroid Storm
Toxicologic exposures
Antibiotic or drug-associated
Enterotoxigenic E. coli (most common cause of watery diarrhea)
Norovirus (often has prominent vomiting)
Campylobacter
Non-typhoidal Salmonella
Enteroaggregative E. coli (EAEC)
Enterotoxigenic Bacteroides fragilis
Giardia lamblia
Cryptosporidiosis
Entamoeba histolytica
Cyclospora
Abdominal aortic aneurysm
Aortocaval fistula
Acute gastroenteritis
Aortoenteric fisulta
Appendicitis (early)
Bowel obstruction
Large bowel obstruction
Ogilvie's syndrome
Small bowel obstruction
Malignant bowel obstruction
Bowel perforation
Gastroparesis
Diabetic ketoacidosis
Hernia
Inflammatory bowel disease
Mesenteric ischemia
Mesenteric vein thrombosis
Pancreatitis
Peritonitis
Sickle cell crisis
Spontaneous bacterial peritonitis
Volvulus
Aortocaval fistula
Large bowel obstruction
Ogilvie's syndrome
Small bowel obstruction
Malignant bowel obstruction
Ogilvie's syndrome
Mesenteric vein thrombosis
Clostridium perfringens
Viral 
Bacillus cereus
Bacterial
Bacteroides fragilis
Campylobacter
Clostridium sordellii
Shigella