Shigellae are Gram-negative, non-motile rods. Shigellae are transmitted by the direct faecal–oral route. As a consequence, food has the potential to be contaminated through the soiled fingers of patients or carriers. The transfer of shigellae by flies breeding on faeces has been established as a very important transmission route during some outbreaks.
Shigella can be found in surface waters and also within contaminated drinking water. Shigella can be classified into four major serological groups.