Scurvy

Wasn't scurvy a problem for 16th century boat-locked sailors? Yes, and scurvy can still be a problem in this most modern and advanced of times. Notably for poor people who cannot afford nutritious food, or anyone with mobility issues such as the sick or the elderly or again the poor. Food deserts. Then there are folks with mental health issues (often the poor, often the elderly, folks with PTSD or eating disorders, etc), or there can be environmental factors such as excess heat or eating at the wrong time of day. Or medical issues where the vitamin C is not absorbed as well as it should be, "malabsorptive states" for a fancy phrase.

Then there's a Goldilocks zone of some, but not too much vitamin C. Going overboard with vitamins can also be bad…