Eligible subjects will be identified by Drs. Faughnan and Gupta. They will subsequently be recruited by the respirology research coordinator (see details below). Next, the hospital pharmacist will provide each subject with a 4-week supply of either norfloxacin 400 mg po bid, or an identical placebo, according to the pre-determined computerized randomization scheme. The pharmacist will be the only person aware of the treatment allocation throughout the study (subjects, research coordinator, treating physicians and outcome assessors will be blinded). After the initial 4-week treatment, there will be a 4-week washout period, after which the pharmacist will provide each subject with a 4-week supply of the alternative agent (crossover) (see figure 2).