Shigellae are small, gram-negative, nonmotile bacilli that are very closely related to E. coli. The four most common Shigella serotypes are S. dysenteriae type 1, S. flexneri, S. boydii, and S. sonnei (which is more prevalent in the industrialized world). Humans are the major reservoir, but Shigella can be found in other higher primates and STEC/EHEC in cows.

• Person-to-person spread via the fecal-oral route is most common for Shigella (and occasionally STEC/EHEC/STEAEC); ingestion of contaminated food and water is the more common route of transmission of STEC/EHEC/STEAEC (and occasionally Shigella).
• The ability of as few as 100 organisms to cause infection helps explain the high rate of secondary household transmission.
• Shiga toxin and Shiga-like toxins produced by some strains of E. coli (including O157:H7) are important factors in disease severity. The toxins target endothelial cells and play a significant role in the microangiopathic complications of Shigella and E. coli infections, such as hemolytic-uremic syndrome (HUS; i.e., Coombs-negative hemolytic anemia, thrombocytopenia, and acute renal failure) and thrombotic thrombocytopenic purpura.
• An analysis of Shigella cases occurring in 1966-1997 revealed an annual incidence of 165 million cases (of which 69% affected children <5 years of age) with 0.5-1.1 million deaths; these numbers have likely decreased since then, but multidrug-resistant strains have emerged.