This is a research study about an experimental (investigational) live attenuated Shigella sonnei vaccine (WRSS1) to find a dose of the vaccine that is safe, tolerable, and develops an immune response. Shigella causes bloody and watery diarrhea, and infants and children living in developing countries experience the greatest consequences of this disease. The WRSS1 vaccine in will be given to healthy toddlers (12-24 months old). The first vaccination will be given to toddlers in the inpatient unit and the second and third doses will be administered on an outpatient basis. A safety evaluation will be preformed after the first dose before enrolling subjects in subsequent cohorts to receive a higher vaccine dose. As of September 2017, the study will be close out after Cohort 1 and enrollment of Cohorts 2, 3 and 4 will not occur.