Prior to the eighteenth century, doctors believed that a person suffering from a chronic illness had an imbalance of body fluids. As a result, they would remove some of the patient’s blood to even out the fluids in a process called bloodletting.

The passage says doctors believed illness came from an imbalance of body fluids and removed blood to even out those fluids. This shows that doctors at the time thought bloodletting would help, which matches “Doctors used to believe that bloodletting was helpful.” The passage does not say whether the practice was later discontinued or is still researched.

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