Viral (e.g. rotavirus)
Bacterial
Campylobacter
Shigella<unk>  (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<unk>  (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<unk> 
Enteroaggregative E. coli (EAEC)
Enterotoxigenic Bacteroides fragilis
Giardia lamblia
Cryptosporidiosis
Entamoeba histolytica
Cyclospora
Salmonella
Viral 
Shigella
Bacterial
Clostridium difficile 
Campylobacter
Clostridium difficile
Campylobacter jejuni