To behave toward someone in a particular way, or to try to cure an illness or improve a condition.
From Old French *traitier* “to deal with, handle,” from Latin *tractare* “to handle, manage,” a frequentative of *trahere* “to pull, drag.” The sense of medical treatment grew from the idea of carefully handling a problem.
Whether you treat someone kindly, treat a stain, or treat a disease, you’re essentially “handling” something with attention. Even a candy “treat” is something handled with special care and pleasure, not just everyday food.
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