The great voyages of discovery, when seafarers such as Magellan and Cook conquered the world’s oceans, brought immense wealth and knowledge to Europe. But they came at a high price. More sailors died of scurvy—more than three times as many—as soldiers were killed in the American Civil War.

Today we know that this terrible ailment, which ravaged both body and mind, was caused by chronic vitamin C deficiency, brought on by lack of fresh fruit and vegetables. But that diagnosis eluded doctors and explorers for centuries, explains Jonathan Lamb in his new book, Scurvy: The Disease of Discovery. (Find out what happened when scurvy struck Christopher Columbus’s crew.)

When National Geographic caught up with the author by phone in