It turns out that we found, then lost, the cure for scurvy well before we eventually identified it as Vitamin C.  There is an incredible write up of that story.

Now, I had been taught in school that scurvy had been conquered in 1747, when the Scottish physician James Lind
proved in one of the first controlled medical experiments that citrus
fruits were an effective cure for the disease. From that point on, we
were told, the Royal Navy had required a daily dose of lime juice to be
mixed in with sailors’ grog, and scurvy ceased to be a problem on long
ocean voyages.

But here was a Royal Navy surgeon in 1911 apparently ignorant of
what caused the disease, or how to cure it. Somehow a highly-trained
group of scientists at the start of the 20th century knew less about
scurvy than the average sea captain in Napoleonic times. Scott left a
base abundantly stocked with fresh meat, fruits, apples, and lime
juice, and headed out on the ice for five months with no protection
against scurvy, all the while confident he was not at risk. What
happened?

It’s really fascinating, definitely worth your time.