There are many mechanisms by which zoonotic pathogens can acquire the ability to spread directly between people, severing the cord that binds them to their reservoir animals. Vibrio cholerae did it by acquiring the ability to produce a toxin.
The toxin was the vibrio's pièce de résistance. Normally, the human digestive system sends food, gastric and pancreatic juice, bile, and various intestinal secretions to the intestines, where cells lining the gut extract nutrients and fluid, leaving behind a solid mass of excreta to expel. The vibrio's toxin altered the biochemistry of the human intestines such that the organ's normal function reversed. Instead of extracting fluids to nourish the body's tissues, the vibrio-colonized gut sucked water and electrolytes out of the body's tissues and flushed them away with the waste.
The toxin allowed the vibrio to accomplish two things essential to its success as a human pathogen. First, it helped the vibrio get rid of its competitors: the massive torrents of fluids sloughed off all the other bacteria in the gut, so that the vibrio (clinging to the gut in its tough micro-colonies) could colonize the organ undisturbed. Second, it assured the vibrio's passage from one victim to another. Even tiny drops of that excreta, on unwashed hands or contaminated food or water, couldcarry the vibrio to new victims. Now, so long as the vibrio could get into a single person and cause disease, it could spread to others, whether or not they exposed themselves to copepods or ingested the vibrio-rich waters of the Sundarbans.
The first pandemic caused by the new pathogens began in the Sundarbans town of Jessore in August 1817 after a heavy rainfall. Brackish water from the sea flooded the area, allowing salty copepod-rich waters to seep into people's farms and homes and wells. V. cholerae slipped into the locals' bodies and colonized their guts. Thanks to the toxin, Vibrio cholerae's basic reproductive number, according to modern mathematicians, ranged from 2 to 6. A single infected person could infect as many as half a dozen others. Within hours, cholera's first victims were being drained alive, each expelling more than fifteen quarts of milky-white liquid stool a day, filling the Sundarbans' streams and waste pits with excreta. It leaked into farmers' wells. Droplets clung to people's hands and clothes. And in each drop, vibrio bacteria swarmed, ready to infect a new host.
The Bengalis called the new disease ola, for “the purge”. It killed people faster than any other disease known to humankind. Ten thousand perished. Within a matter of months, the new plague held nearly two hundred thousand square miles of Bengal in its grip.
Cholera had made its debut.
— Pandemic: Tracking Contagions, from Cholera to Ebola and Beyond (Sonia Shah)
