Med Quizzes

Infectious Causes of Diarrhea

Infectious Diarrhea Cheatsheet

Key distinctions: Watery vs bloody diarrhea, fever pattern, and source narrow the diagnosis fast. STEC and Shigella both cause bloody diarrhea — but only STEC forbids antibiotics. Campylobacter and STEC both cause HUS-spectrum complications — but via different toxins. Salmonella and Shigella both cause high fever — but Salmonella requires far fewer antibiotics.

Organism	Age / Setting	Diarrhea Type	Source / Trigger	Key Feature / Sequelae	Treatment
Rotavirus	Age <2 yr (peak 6–24 mo)	Watery, non-bloody (3–8 days)	Fecal-oral; stable on fomites; daycare outbreaks	Vaccine-preventable: RotaTeq / Rotarix oral series (<8 months)	Supportive + ORS; no antivirals
Campylobacter	All ages; most common bacterial GI pathogen in US	Bloody, mucoid; severe crampy abdominal pain	Undercooked poultry; unpasteurized milk	Post-infectious Guillain-Barré syndrome (ascending paralysis, 2–4 wks)	Azithromycin (first-line in children)
STEC / O157:H7	All ages; no fever/travel; isolated cases common	Watery → bloody within 1–3 days	Undercooked ground beef; farm animal contact	HUS: hemolytic anemia + thrombocytopenia + AKI (1–2 wks post-diarrhea)	AVOID antibiotics + antidiarrheals; aggressive IV fluids
Shigella	Children 1–4 yr; daycare / institutional settings	Bloody, mucoid; tenesmus	Fecal-oral; lowest infectious dose (<10 organisms)	High fever; febrile seizures in young children	Always treat: azithromycin or TMP-SMX
Salmonella	Severe in infants <3 mo, elderly, sickle cell patients	Watery or bloody; fever, nausea, vomiting	Poultry, raw eggs, reptiles (turtles)	Bacteremia → osteomyelitis (esp. in sickle cell disease)	Avoid in uncomplicated (prolongs shedding); treat if <3 mo, bacteremia, or immunocompromised
C. difficile	Elderly, hospitalized, nursing home residents	Profuse watery; pseudomembranous colitis (severe)	Prior antibiotics: clindamycin, ampicillin, fluoroquinolones	Toxins A + B; dx by fecal PCR/toxin assay; leukocytosis	Oral vancomycin or fidaxomicin (first-line); metronidazole (mild only)

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Infectious Causes of Diarrhea

A Rotavirus

B Campylobacter

C STEC / E. coli O157:H7

D Shigella

E Salmonella

F C. difficile