Food, mostly produce, can be contaminated with shigella through unclean or unsanitary food handling practices.
Specifically, by being handled by food preparers who have shit under their fingernails. Also, produce can become contaminated by being irrigated with shitty water or by being shit upon by animals while still growing in the ground.
Shegellosis will make you shit uncontrollably, and your shit will be extremely bloody and painful. Immediate medical attention is required.