It is apparent from epidemiologic investigations that as children age into their teens and adulthood they are naturally protected from Shigella and this is suspected to be related to multiple exposures to other antigens than the O-antigen over time. To understand this phenomenon more clearly, this sero-epidemiological study will follow patients naturally infected with both Shigella and ETEC over time as well as healthy endemic adults to explore their immune response to antigens other than O-antigen. Although prophylactic vaccination is an effective way to reduce the huge disease burden associated with diarrhoea caused by enteric pathogens, many attempts to develop vaccines for shigellosis and ETEC infections have failed and a number of current approaches are too complex and costly to provide an adequate solution for low- and middle-income countries. By identifying immune responses to a plethora of specific bacterial antigens, these data will inform both the potential value of the ShigETEC vaccine that lacks O-antigen as well as will  help identify other antigens that may be needed in future generations of the ShigETEC vaccine to improve its efficacy. To accomplish this goal, both mucosal and systemic responses in adult (18-45 years) and paediatric (up to 5 years) patients with shigellosis or ETEC diarrhoea as well as healthy adult controls (18-45 years) in Bangladesh will be evaluated.