Yan and Fabini published a hypothesis paper entitled "The out-of-the-delta hypothesis: dense human populations in low-lying river deltas served as agents for the evolution of a deadly pathogen" in Frontiers in Microbiology. Dr. Munirul Alam, a collaborator from the International Centre for Diarrhoeal Disease Research, Bangladesh, is also co-author of the paper.
The authors propose that the unique human and physical geography of the Ganges Delta caused the naturally-occurring and nonpathogenic Vibrio cholerae to evolve into a deadly human pathogen. This is mainly due to the regular ingestion of V. cholerae through consumption of brackish water by the inhabitants of the Ganges Delta area and the subsequent release of V. cholerae back into the environment, therefore creating a continuous selection pressure for V. cholerae to adapt to life in the human gut.
The red and blue areas on the world map each house 5% of the world's population (from Boucher et al., 2015; map constructed by Max Galka, used with permission)