The purpose of this study is to identify the human intestinal microbiota (microbes that live inside and on human bodies) in healthy adults over a 6-month period and to study the effect of an antibiotic on the intestinal microbiota. Participants will include up to 60 healthy adult subjects, ages 18-45 years, from the Baltimore and University of Maryland communities. Study procedures will include providing multiple stool samples throughout the study. Participants will take a licensed antibiotic, Ciprofloxacin, for 3 days. Participants may be involved in study related procedures for up to 7 months.