Specimens collected in recruiting clinics will be stored in a 2-8°C refrigerator (overnight, if required). Specimens collected during home visits will be stored at room temperature or if necessary (i.e. in hot weather) in a cool box with at least 2 icepacks immediately after collection. Before the end of the day of a home visit, the study nurse will take any collected samples to the nearest recruitment clinic for storage in a 4°C refrigerator (overnight, if required) or directly to the Department of Microbiology, HKU. Samples stored at 2-8°C in recruiting clinics will be delivered to the Department of Microbiology, HKU as soon as possible, via courier and maintained at 2-8°C en route. Samples will be eluted and cryopreserved at minus 70°C at the destination.