Inclusion Criteria:  Individuals aged 18 and above; Subjects who are mentally capable to participate in the study and provide informed consent; Subjects who can communicate in Chinese or English; Subjects who are ambulatory and do not have difficulties travelling to the clinics for follow-up; Subjects who do not have plans to leave Hong Kong in the subsequent two years after recruitment; and Subjects who agree to give informed consent voluntarily.  Exclusion Criteria:  Subjects who are unable to receive oral fluids; Subjects who have received surgery involving the intestine within past 30 days; Subjects who are pregnant or breastfeeding; and Subjects who are immunocompromised, e.g. on cancer treatment, bone marrow/organ transplant, immune deficiency, poorly controlled HIV/AIDS.