Deficiency
Etymology
From Latin “deficere” (to fail or fall short), from “de-” (down, away) and “facere” (to do or make) — to leave undone, to run out. The word frames a distinct and once-mysterious kind of disease: not something present that should not be, but something missing that should be. Scurvy and rickets were deficiencies long before anyone had the word for why.
noun · /dih-FISH-uhn-see/
A harmful shortage of something the body needs, such as a vitamin, mineral, or hormone.
What it names
A whole category of illness caused not by an invader or a growth but by simple absence — the trouble that comes from not having enough of an essential thing.
Example
Her constant tiredness turned out to be an iron deficiency, corrected in weeks once she started supplements.
Related idea
A deficiency is disease by absence; an infection is disease by invasion. The two demand opposite remedies — you cure a deficiency by adding what is missing, and an infection by removing what has intruded — which is why diagnosing the right kind matters so much.