Epidemiology

Etiology

Pathogen
- Salmonella enterica serotype Typhi: typhoid fever
  - Gram-negative rod
  - Facultative anaerobe with peritrichous flagella
  - Produces hydrogen sulfide (H2S) on TSI agar
  - Oxidase-negative
  - Cannot ferment lactose
- Salmonella enterica serotype Paratyphi: paratyphoid fever
Reservoir
- Salmonella enterica serotype Typhi: humans
- Other Salmonella species: humans and animals
Transmission: fecal-oral

Oral uptake of pathogen: A relatively large number of organisms (∼ 105) is needed to cause infection (high infective dose), unlike, e.g., in Shigella infection, where as few as ∼ 10 organisms suffice to infect the host.
Migration into the Peyer patches of the distal ileum: If the pathogen manages to reach the distal ileum, it migrates via M cells through the epithelium and into the Peyer patches.
Infection of macrophages → nonspecific symptoms
Spread from macrophages to the bloodstream → septicemia → systemic disease
Migration back to intestine → excretion in feces

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