Enteroaggregative E. coli (EAEC) is a bacterium that can cause diarrhea. The purposes of this study are to: determine how much EAEC is needed to cause diarrhea in a healthy person, determine if a genetic factor is important in causing diarrhea, and to see how the body's defenses control EAEC. Participants include 25 healthy adults, ages 18-40. Volunteers will be assigned to 1 of 4 dose levels in groups of 5 volunteers each. One volunteer in each group will receive a sodium bicarbonate placebo solution. Volunteers will be admitted to the University Clinical Research Unit for up to 8 days. Volunteers will receive therapy with levofloxacin to treat the infection either once they develop diarrhea or at Day 5 if they remain asymptomatic. Study procedures will include saliva, blood, and fecal sample collection. An optional study procedure will include an intestinal biopsy. Participants will be involved in study related procedures for up to 223 days.