This is a single-university center (University of Toronto), randomized, controlled pilot study with a crossover design. The intervention is exposure to norfloxacin (400 mg po bid) for a 4-week period, compared to an identical placebo treatment. In the crossover design, all subjects will receive both norfloxacin and the placebo medication, but the order of treatment will be randomized, as detailed in the study maneuver, below.