The ultimate source of Shigella bacteria is the infected excrement of a previously infected individual. That infectious material is spread to new cases by person-to-person contact or via contaminated food or water. A new case of bacillary dysentery occurs after the organism is ingested.
Shigella bacteria are in a small group of germs (that also includes E. coli O157:H7 and Cryptosporidium) that can infect the gut after the ingestion of relatively few organisms. Volunteer experiments have demonstrated that shigellosis can occur after ingestion of fewer than 200 bacteria,1 a very small amount. This is why bacillary dysentery is the most communicable of the bacterial-induced diarrheas and that is most commonly transmitted by person-to person contact.

Approximately 20% of cases of shigellosis are transmitted via contaminated food or water.2 Generally, the food preparer is the individual who contaminates the food, but food may also become contaminated previously during processing. Contamination of drinking water by Shigella is a problem that generally only occurs in the developing world.3