Vibrio bacteria and copepods proliferated in warm, brackish coastal waters, where fresh and salty waters met, such as in the Sundarbans, an expansive wetlands at the mouth of the world's largest bay, the Bay of Bengal. This was a netherworld of land and sea long hostile to human penetration. Every day, the Bay of Bengal's salty tides rushed over the Sundarbans' low-lying mangrove forests and mudflats, pushing seawater as far as five hundred miles inland, creating temporary islands of high ground, called chars, that daily rose and vanished with the tides. Cyclones, poisonous snakes, crocodiles, Javan rhinoceroses, wild buffalo, and even Bengal tigers stalked the swamps. The Mughal emperors who ruled the Indian subcontinent up until the seventeenth century prudently left the Sundarbans alone. Nineteenth-century commentators called it âa sort of drowned land, covered with jungle, smitten by malaria, and infested by wild beasts,â and possessed of an âevil fertilityâ.
But then, in the 1760s, the East India Company took over Bengal and with it the Sundarbans. English settlers, tiger hunters, and colonists streamed into the wetlands. They recruited thousands of locals to chop down the mangroves, build embankments, and plant rice. Within fifty years, nearly eight hundred square miles of Sundarbans forests had been razed. Over the course of the 1800s, human habitations would sprawl over 90 percent of the once untouched, impenetrable, and copepod-rich Sundarbans.
Contact between human and vibrio-infested copepod had probably never been quite so intense as in these newly conquered tropical wetlands. Sundarbans farmers and fishermen lived in a world semisubmerged in the half-salty water in which vibrio bacteria thrived. It wouldn't have been particularly difficult for the vibrio to penetrate the human body. A fisherman who splashed his face with water by the side of a boat, say, or a villager drinking from a well corroded with a few ounces of floodwaters, could easily ingest a few invisible copepods. Each one might be infested with as many as seven thousand vibrios.
This intimate contact allowed Vibrio cholerae to âspill overâ or âjumpâ into our bodies. The bacteria wouldn't have found a particularly welcome reception there, at first. Human defenses are designed to repel such intrusions, from the acid environs of our stomachs, which neutralize most bacteria, and the competitive wrath of the microbes that inhabit our gut to the constantly patrolling cells of the immune system. But in time, V. cholerae adapted to the human bodies to which it was repeatedly exposed. It acquired, for example, a long, hairlike filament at its tail that improved its ability to bond to other vibrio cells. Endowed with the filament, the vibrio could form tough microcolonies that could stick to the lining of the human gut like scum on a shower curtain.
Vibrio cholerae became what's known as a zoonosis, from the Greek zoon for âanimalâ and nosos for âdiseaseâ. It was an animal microbe that could infect humans. But V. cholerae wasn't a pandemic killer yet.
 â Pandemic: Tracking Contagions, from Cholera to Ebola and Beyond (Sonia Shah)












