How is Shigella infection spread?
Human cases of Shigella are typically acquired through contact with infected persons or ingestion of food contaminated with Shigella bacteria.
As previously noted, Shigella species are transmitted by the fecal-oral route, and most infections are transmitted from person to person, reflecting the low infectious dose.  As also noted, as few as ten Shigella bacteria can result in clinical infection. 
Where persons who are infected may be present, the risk of transmission and infection increases with poor hand hygiene, ingestion of contaminated food or water, inadequate sanitation and toileting, overcrowding, and sexual contact. [4, 14, 21, 24, 28] Shigella bacteria are present in the stools of infected persons while they are sick and for up to a week or two afterwards. [11, 16, 17] It is estimated that up 80% of all infection is the result of person-to-person transmission. 
Because of its quite common person-to-person spread, shigellosis has long been associated with outbreaks in daycare centers, nursing homes, institutional settings (like prison), and cruise ships. [11, 14, 17, 23, 24] Explaining the significance of daycare centers as a source of Shigella infection, one well-respected study explains as follows:
High shigellosis rates in children are attributable to several factors. Young children are unable to practice good personal hygiene and have not yet acquired immunity to S. sonnei. The infectious dose is as low as 10–200 organisms, and person-to-person transmission is highly effective. Day-care centers play an important role in the person-to-person spread of shigellosis and its subsequent dissemination in communities. Inadequate hand washing, diapering practices, and fecal contamination of water-play areas, such as kiddie pools, have been associated with S. sonnei transmission in day-care centers. 
Several studies have demonstrated an increased frequency of shigellosis cases in young adult men residing in urban settings who have little, if any, exposure to these traditionally recognized risk groups. [4, 36] Although some of these studies indicated that sex between men can be a risk-factor, most of these studies occurred before the HIV epidemic. [4, 23]
Shigella infections also may be acquired from eating contaminated food. A study published in 2010 estimated more than one-third of U.S. shigellosis cases annually might be caused by the consumption of contaminated food.  In the United States, incidence of foodborne illness is documented through FoodNet, a reporting system used by public health agencies that captures foodborne illness in over 13% of the population. [8, 9] Of the 10 pathogens tracked by FoodNet, Salmonella, Campylobacter, and Shigella are responsible for most cases of foodborne illness.  An estimated 20% of the total number of cases of shigellosis involved food as the vehicle of transmission. 
In one oft-cited study summarizing food-related illness and death in the United States, the following synopsis is set forth at the end, summarizing Shigella.
Reported cases: Outbreak-related cases based on reports to CDC, 1983-1992. Passive surveillance estimate based on average number of cases reported annually to CDC, 1992-1997. Active surveillance estimate based on extrapolation of average 1996-1997 FoodNet rate to the 1997 U.S. population.
Total cases: Because
Shigella frequently causes bloody diarrhea, total cases assumed to be 20 times the number of reported cases, based on similarity to E. coli O157:H7.
Hospitalization rate: Based on hospitalization rate for culture-confirmed cases reported to FoodNet, 1996-1997.
Case-fatality rate: Average case-fatality rate among cases reported to FoodNet, 1996-1997 (23,24). Percent foodborne: Assumed to be 20%. Although most cases are due to person-to-person transmission (60), foodborne outbreaks are responsible for a substantial number of cases 
According to the CDC, Shigella is the third most common pathogen transmitted through food. In FoodNet surveillance areas in 2008, the rate of Shigella was 6.6 per 100,000 population, exceeded only by Salmonella (15.2/100,000) and Campylobacter (12.7/100,000).  During 2006, public health officials reported a total of 1,270 foodborne-related outbreaks from 48 states in the U.S.  Although Shigella was responsible for only 10 (1%) of those outbreaks, 183 confirmed cases of shigellosis were nonetheless reported.  This reporting rate contrasts with an average of 659 cases annually in the previous five years, making it potentially an aberration or outlier.
Shigella is also responsible for a substantial portion of foodborne outbreaks on cruise ships. [16, 34] In a review of cruise ship outbreaks worldwide over several years, 16% of outbreaks were attributed to Shigella, affecting over 2,000 passengers.  Sanitation violations related to food handling and communicable disease have decreased substantially, however, over the past 15 years.