It is spread by the fecal-oral route and is highly communicable. It is estimated that less than 100 Shigella can start an infection, whereas it can take tens of millions of, for instance, cholera vibrios to cause disease.
Transmission is by contaminated fingers, food, or water.
Children in group care are considered to be at high risk, because of poor hygiene and close contact. The organisms multiply in the small intestine, then invade the large intestine. Disease is caused by the invasion of the colonic epithelial cells and multiplication of the organisms within these cells and the lamina propria with attendant cell death and tissue destruction. This produces acute inflammation and ulceration of the mucosa. Shigella dysenteriae, in contrast to S. flexneri, S. sonnei, and S. boydii, can also cause cell death by the production of Shiga toxin, an A-B type toxin with 1 A subunit and 5 B subunits.