- Shigella: Causes dysentery (bloody diarrhea); very low infectious dose (10-100 organisms).
- Enterohemorrhagic E. coli (EHEC O157:H7): Can cause bloody diarrhea and Hemolytic Uremic Syndrome (HUS); low infectious dose.
- Campylobacter jejuni: Common cause of bloody diarrhea; associated with Guillain-Barré syndrome; low infectious dose.
- Giardia lamblia: Protozoan causing fatty diarrhea (giardiasis); infectious with few cysts.
- Cryptosporidium parvum: Protozoan causing watery diarrhea, severe in immunocompromised; very low infectious dose.
- Francisella tularensis: Causes tularemia; highly infectious with very few organisms (potential bioterrorism agent).
- Coxiella burnetii: Causes Q fever; extremely infectious (as few as one organism inhaled).
- Norovirus: Highly contagious cause of viral gastroenteritis outbreaks; very low infectious dose.
- Rotavirus: Major cause of severe diarrhea in young children; low infectious dose.