Cholera Gave a British Doctor the Idea for IVs

Patients need fluid, somehow.

Physicians have been trying to stick fluids into patients—or experimental animals—for hundreds of years. In 1656, Sir Christopher Wren wanted to see what would happen if he took a quill and a pig's bladder and used it to send intoxicants—beer and wine and opium—directly into a dog's blood. (The dog was intoxicated.) That was just the beginning of a series of 17th-century experiments to infuse blood with other substances—mostly blood. Experimentally minded researchers and physicians inject animal blood into other animals, animal blood into humans, human blood into other humans.
At the same time, often physicians were trying to get blood out of sick humans, through bloodletting. This was a common practice in the 1830s, when a cholera epidemic hit England. The Royal College of Physicians, though, wanted to look more closely at how cholera affected its victims's bodies, and it sent a young Scottish physician, William O'Shaughnessy, to investigate.
Building on work in Russia, which had shown that cholera patients's blood was much more concentrated than normal—it had 30 percent less water in it—O'Shaughnessy reported back to the college that, to help cholera patients, doctors should attempt to bring blood back to its natural state. Adding water didn't seem to be enough, though: The blood needed the right saline content, too.
O'Shaughnessy didn't say exactly how to restore the blood though; he suggested "absorption… imbibition or…the injection of aqueous fluid into the veins." Another British physician, Thomas Latta, picked up on this idea and started infusing cholera patients with saline solution. And it seemed like it might work. Here's his description of his first IV treatment:

She had apparently reached the last moments of her earthly existence ... I feared that I should be unable to get my apparatus ready ere she expired.  ... ounce after ounce was injected, but no visible change was produced. Still persevering, I thought she began to breathe less laboriously, soon the sharpened features, and sunken eye and fallen jaw, pale and cold ... began to glow with returning animation; the pulse, which had long ceased, returned to the wrist; at first small and quick, by degrees it became more and more distinct ... and in the short space of half an hour, when six pints had been injected ... her extremities were warm, and every feature bore the aspect of comfort and health.

Within a few hours, that patient's symptoms returned. But the idea was correct, and Latta's next patients did better. Another physician tried the strategy, too, and had 25 patients, out of 156, recover.
It wasn't a perfect cure. But it helped, and as knowledge of blood improved, so did transfusion of both blood and saline. Today most patients in U.S. hospitals receive some sort of IV treatment.