Shigella bacteria cause human illness when they are ingested, and can lead to Shigella infection, or shigellosis, through various modes of transmission, including through food and water sources, animal-to-human contact, and person-to-person contact in daycares and other settings.
The introduction of pasteurization greatly reduced the number of foodborne illness outbreaks associated with milk and other dairy sources, but the consumption of raw milk and unpasteurized cheeses remains a risk factor for Shigella infection. Shigella and other pathogens are shed in the feces of livestock such as cows and goats and can contaminate milk during the milking process.
Fresh fruits and vegetables can become contaminated before or after harvest. Water intended for recreation (e.g., pools, shallow lakes) and for human consumption can also become contaminated. When lakes become contaminated it may be several weeks or months before water quality conditions to improve or return to normal. Proper chlorination kills Shigella bacteria in pools and municipal water systems.
Person-to-person transmission of Shigella occurs through a fecal-oral route, and is particularly common among infants and young children due to their unrefined hygienic practices. Person-to-person transmission of Shigella has also been known to occur between infected individuals and their caregivers, and between infected food handlers and people who consume the food they prepare.