An interdisciplinary curriculum unit that used Web GIS mapping to investigate malaria disease patterns and spread in relation to the environment for a high school Advanced Placement Environmental Science course was developed. A feasibility study was conducted to investigate the efficacy of the unit to promote geospatial thinking and reasoning skills and content understandings. Results revealed increased content understandings and significant effect sizes for all three geospatial thinking and reasoning subscales--inferences, relationships, and reasoning. The findings provide support that Web GIS, with appropriate curriculum design can improve both learning outcomes and geospatial thinking and reasoning skills.