BRAC Burundi ORS formulations OTEP Public health & preventive medicine VIP Vibrio cholerae adaptive antibody cellular cholera cholera control cholera patients community-based care diarrhea effectiveness efficacy environmental parameters history hot spot hyponatremia hyponatremic seizures hyponatremic sequelae immunity innate memory mucosal mucosal immunity non-cholera acute watery diarrheas (AWDs) non-cholera dehydrating diarrheas