Cholera epidemics in India usually got started during monsoons. During the monsoon, warm rain fell and raised the water temperature in the ocean near the shore. Streams run into the ocean and bring food from the land, and algae grow in the warm water. The cholera germs eat this food and algae. And they like the warm water, so the germs reproduce and make a lot of cholera germs. Sea creatures like crabs and clams drink the cholera germs. So when people catch and eat the crabs and clams, they get cholera too.
But the reason you get big cholera epidemics is when people pass cholera to each other by drinking water with cholera germs in it. When people had cholera, the cholera germs were in their poop. If your city didn’t have a good way to separate sewage from drinking water, then other people would catch cholera. Cholera germs give you terrible diarrhea and vomiting. More than half the people who got it died from dehydration (not enough water inside them). But if you were healthy otherwise, and good nurses took care of you, and made sure you had plenty of clean water and some sugar and salt, then you would usually live. Children, old people, and especially poor people who didn’t get enough to eat or good nursing, often died of cholera.
In 1817 AD, there was another big cholera epidemic in India. The eruption of Mt. Tambora in Indonesia caused bad weather in India and seems to have made the epidemic worse. But this time, the cholera germs seem to have evolved to be easier to spread. Trading ships and traders over land began to spread cholera out of India.
By 1820, thousands of people were dying of cholera in Russia. Then cholera spread to China, Iran, the Ottoman Empire, and Sudan. By 1827, people were dying of cholera in Europe and North America. The philosopher Hegel may have died of cholera, for example. In 1839, cholera reached South America, and in 1863 central and southern Africa.
In the late 1800s, big cities in Europe and the United States began to fight cholera. Governments built sewage and water systems to separate poop from clean drinking water. This pretty much stopped cholera. It also stopped dysentery in general, which had killed many babies before. The last big cholera epidemic in the United States was in 1911. But people are dying of cholera right now in Haiti, because United Nations aid workers brought it with them to Haiti from Pakistan.