The structure and function of the mammalian gut vary by region, yet why inflammatory diseases manifest in specific regions and not others remains unclear. We use a TNF-overexpressing Crohn's disease (CD) model (Tnf ΔARE/+ ), which typically presents in the terminal ileum (TI), to investigate how environmental factors interact with the host's immune susceptibility to drive region-specific disease. We identified Chlamydia muridarum , an intracellular bacterium and murine counterpart to the human sexually transmitted C. trachomatis , as necessary and sufficient to trigger disease manifestation in the ascending colon (AC), another common site of human CD. Disease manifestation in the AC depends on indoleamine 2,3-dioxygenase (IDO1) activity induced by hypersensitive surface secretory cells in genetically susceptible hosts. Single-cell and microbial analyses of human specimens also implicates this pathobiont-epithelial IDO1 pathway in patients with a history of CD in the AC. Our findings demonstrate that genetic and microbial factors can independently drive region-specific disease and provide a unique model to study CD specific to the AC.