Infectious: noninflammatory (nonbloody)
- Viruses: Norwalk virus, rotavirus, adenoviruses, astrovirus, coronavirus
- Preformed toxin (food poisoning): Staphylococcus aureus, Bacillus cereus, Clostridium perfringens
- Toxin production: enterotoxigenic E. coli, Vibrio cholerae, Vibrio parahaemolyticus
- Protozoa: Giardia lamblia, Cryptosporidium, Cyclospora, Isospora
Infectious: invasive or inflammatory
- Shigella, Salmonella, Campylobacter, enteroinvasive E. coli, E. coli O157:H7, Yersinia enterocolitica, Clostridium difficile (e.g., pseudomembranous colitis), Entamoeba histolytica, Neisseria gonorrhoeae, Listeria monocytogenes
Associated with unprotected anal intercourse
- Neisseria gonorrhoeae, syphilis, lymphogranuloma venereum, herpes simplex
Noninfectious
- Drug reaction, especially antibiotics
- Ulcerative colitis, Crohn's disease (inflammatory)
- Ischemic colitis (inflammatory)
- Fecal impaction (stool may leak around impaction)
- Laxative abuse
- Radiation colitis (inflammatory)
- Emotional stress