This thesis is an investigation of the burden of acute gastrointestinal illness (GI) in Galvez, Argentina and the Metropolitan region, Chile. A cross-sectional survey was administered door-to-door to 2915 and 6047 randomly selected residents of Galvez and the Metropolitan region, respectively. Two time periods, coinciding with high and low GI seasons, were selected for survey administration. Shorter (i.e., 7 and 15 day) and longer (i.e., 30 day) recall periods were incorporated in the surveys. The surveys were designed to determine the distribution and population-level burden of GI, identify risk factors including food and water consumption and food safety behaviours, estimate pathogen-specific community-level incidence rates, and evaluate the effect of different recall periods on GI incidence rates in population-level burden of GI studies. High response rates of 61% and 76% were obtained in Galvez and the Metropolitan region, respectively. The annual incidence rate of GI in the Metropolitan region ranged from 0.98 to 2.3 episodes per person-year, for a 30-day and a 7-day recall period, respectively, and the annual incidence rate of diarrhea in Galvez, ranged from 0.46 to 1.68 episodes per person-year for a 30-day and a 7-day recall period, respectively. In general, little difference in the rate of GI was seen between men and women, while children 0-4 and youths 10-19 had higher rates of GI in both Galvez and the Metropolitan region. In the Metropolitan region, GI was associated with consumption of undercooked poultry, undercooked beef, cheese made from unpasteurized milk and higher water consumption, though causality cannot be inferred due to the different recall periods used for risk factors versus GI symptoms and the cross-sectional study methodology. The estimated community-level, pathogen-specific incidence rates for 'Salmonella, Campylobacter' and 'Shigella' infections were greater than the laboratory confirmed incidence rates, likely due to ill residents not seeking medical care, not submitting a stool sample and some laboratories not routinely testing for certain enteric pathogens. This research has demonstrated the significant burden of GI in these South American communities that is not captured by traditional surveillance and outbreak registries, and has highlighted risk factors that can inform public health policy and prevention activities.