This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
For most of medical history, drawing blood from a sick patient was not a fringe remedy. It was the treatment, prescribed by the most respected physicians in the world for the widest possible range of complaints, from fevers to headaches to melancholy to plague. Its authority rested on humoral theory, the idea, laid out in the Hippocratic corpus and later organized into a complete medical system by Galen, that health was a balance of four bodily humors and that most disease reflected an excess, commonly of blood itself. Removing blood, by opening a vein or applying leeches, was therefore not a side treatment but the logical center of therapeutic practice, and it stayed there, essentially unchallenged as a system, for close to two thousand years across the Greco-Roman, medieval and early modern medical worlds.
What ended its dominance was not a new theory of disease so much as a new way of asking whether a treatment worked at all. The French physician Pierre Charles Alexandre Louis spent years at the Hopital de la Charite in Paris comparing the recorded outcomes of pneumonia patients who were bled early, bled later, or barely bled, publishing his comparative findings in the 1830s. He found no advantage for early or aggressive bleeding, and beyond that specific result, he argued for something more consequential: that a treatment's effect could only be trusted once it had been checked across enough patients to separate a real pattern from the ordinary unpredictability of disease. This numerical method, one of the direct ancestors of the modern clinical trial, gave doctors for the first time a way to test a treatment that centuries of theoretical authority had simply assumed worked.
Bloodletting did not disappear overnight, and it was not wholly wrong to reach for. It survives today, on narrow and genuinely evidence-based grounds, as therapeutic phlebotomy for a small number of conditions, hemochromatosis and polycythemia among them, where excess iron or excess red blood cells are the literal problem a blood draw literally treats. What collapsed across the nineteenth century was its use as a general-purpose cure grounded in humoral balance, which medical historians now regard as ineffective for the great majority of the conditions it was once prescribed for, and harmful for some of the sickest patients it was used on most aggressively. The lesson historians draw from it is not that its physicians were foolish, most were the most careful and well-read practitioners of their age, but that even careful reasoning from a coherent theory is not a substitute for checking, and that the checking itself had to be invented before it could be applied.