Infectious:
Salmonella
Shigella
Shiga toxin-producing E. coli
E. coli 0157:H7
Entamoeba histolytica
Yersinia
Vibrio
C. difficile 
Non-infectious:
Inflammatory bowel disease
GI bleed
Adrenal insufficiency
Mesenteric ischemia
Thyroid storm
Toxicologic exposure
Radiation syndrome
Salmonella
Shigella
Shiga toxin-producing E. coli
E. coli 0157:H7
Entamoeba histolytica
Yersinia
Vibrio
C. difficile
Inflammatory bowel disease
GI bleed
Adrenal insufficiency
Mesenteric ischemia
Thyroid storm
Toxicologic exposure
Radiation syndrome
Viral (e.g. rotavirus)
Bacterial<unk> 
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<unk> 
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)<unk> 
Non-typhoidal Salmonella
Enteroaggregative E. coli (EAEC)
Enterotoxigenic Bacteroides fragilis
Giardia lamblia
Cryptosporidiosis
Entamoeba histolytica
Cyclospora
Campylobacter jejuni
Infectious:
Salmonella
Yersinia enterocolitica
Shigella
Clostridium difficile 
Shiga toxin-producing E. coli
Clostridium difficile