What critical discovery did Robert Koch make regarding cholera?

Robert Koch made a pivotal contribution to our understanding of cholera by isolating the cholera bacillus, Vibrio cholerae. This discovery was foundational in microbiology and public health because it provided clear evidence linking a specific microorganism to a specific disease. By identifying the causative agent of cholera, Koch's work laid the groundwork for subsequent research on infectious diseases and helped shape disease management strategies that focused on pathogen identification. This advance was pivotal in developing methods to combat cholera outbreaks, leading to improved public health measures.

The other options, while significant in the history of disease understanding and epidemiology, do not specifically relate to Koch's direct findings about cholera. The discovery of the cholera vaccine is associated with different researchers and efforts based on Koch's earlier work. Conducting the first epidemiological study is often attributed to John Snow in relation to cholera, emphasizing waterborne transmission. Lastly, while Koch's research contributed to understanding waterborne diseases, the specific identification of polluted water sources was not his primary discovery regarding cholera itself.