 A nasal swab and stool sample will be requested at four different time points: 1 within 48 hours of the first antibiotic dose; 2 7-14 days after starting antibiotics; 3 3 months after starting antibiotics; and 4 1 year after starting antibiotics. At each time point, additional information will be collected: previous overseas travel, previous hospital admissions, household member who has been admitted to hospital overseas, other antibiotic use, other infections, medical visits or hospital admissions. These samples are optional and do not affect participation in the study. 