Why did sailors on long voyages used to get scurvy?

Sailors on long voyages got scurvy because their diet, month after month of ship's staples with no fresh fruit or vegetables, left them without vitamin C, which almost every other living thing can make for itself but humans cannot. Without it, the body can't make the building blocks of collagen, the protein that holds blood vessels, skin, and connective tissue together, so capillaries become fragile, causing abnormal bleeding, bruising, and internal hemorrhaging. During the Age of Sail, it was simply assumed that half the crew on a major voyage of exploration would die of scurvy, and researchers estimate the disease killed at least two million sailors between 1500 and 1800. On Vasco da Gama's 1499 voyage, 116 of his 170 crew died; Magellan lost 208 of his 230 men in 1520, in both cases mainly to scurvy.

The fix was hiding in plain sight for centuries, but it took a long time to be believed. In 1747, naval surgeon James Lind ran one of medicine's first controlled clinical trials aboard HMS Salisbury, comparing suggested cures such as hard cider, vinegar, seawater, and oranges and lemons, and concluded that oranges and lemons were 'the most effectual remedies for this distemper at sea.' Yet scurvy kept killing sailors for decades afterward: during the entire 18th century, it killed more British sailors than enemy action did, and of the 184,899 sailors the Royal Navy enlisted during the Seven Years' War, 133,708 were lost to sickness, discharge, or desertion, with scurvy the leading cause.

Even after citrus was finally adopted, the fix nearly got undone by accident. When West Indian limes later replaced lemons, they had less vitamin C to begin with, and worse, the juice was exposed to light and air and piped through copper tubing, which reduced its vitamin C even further. The ration kept the old reputation (it's the reason British sailors became known as 'limeys') without doing nearly as much good.

A key step toward the real cause came in 1907, almost by accident. Two Norwegian physicians, Axel Holst and Theodor Frølich, were trying to study a different disease, beriberi, and needed a small test animal to replace the pigeons they'd been using. They fed guinea pigs the same grain-based diet that gave pigeons beriberi, and were startled when the guinea pigs developed scurvy instead. It turned out pigeons simply can't get scurvy, since they can make their own vitamin C internally. Guinea pigs gave scientists an animal model of the disease, before the very idea of vitamins had even been put forward.