Regarding scurvy, I read an essay that said the cure actually needed to be re-discovered. Someone had decided to load the ships with a related citrus fruit that

Regarding scurvy, I read an essay that said the cure actually needed to be re-discovered. Someone had decided to load the ships with a related citrus fruit that had much less vitamin C in it, leading to doubts about the hypothesis since it meant sailors would get scurvy despite getting citrus. This got cleared up later, and IIRC later yet someone figured out the critical ingredient.