V. cholerae secrete choleragen to grow and escape human intestines, however choleragen does not work on other mammals, why so? Why didn't it evolve a general mammal affecting choleragen? This question is a continuation of the last comment on the only answer for my previous question, How does Vibrio cholerae benefit from infecting its host? Kindly refer to the answer and comments in that question as well.
Vibrio cholerae originated in the Sundarbans, a very large wetland at the mouth of the Bay of Bengal. Although it can exist as free living, it is much more fecund when associated with tiny crustaceans called copepods. In 1760, the East India Company settled and sprawled, hewing the mangroves and planting rice. So, that was the first opportunity of note introducing V. cholerae to a major species of land animal, humans. The frequent contact with a sizable population of humans, plus time, allowed V. cholerae to evolve the traits to become a parasite.
For example, a hair-like filament evolved which allowed the cells to clump together forming colonies that could stick to the gut. At that point, perhaps with additional traits, V. cholerae became a zoonosis. That is, a disease we acquire from animals, copepods in this case, but not directly from other humans. Further evolution over time gave it the traits to spread directly between humans making it a true human pathogen.
So, humans were the species that became available and V. cholerae evolved to take advantage.
This is a minute fraction of the whole story as related in the 2016 book Pandemic by Sonia Shah. She is a science writer, the book is extremely well referenced. Since this is your second question on this, I suspect you may well be interested in the book.