So there really are diseases that can be treated through bleeding? Yes, says Bill Schutt in Dark Banquet: blood and the curious lives of blood-feeding creatures, and he goes through a few – porphyria, for instance. “Porphyria (from the Greek word for ‘purple’) is a disease of the blood that results from the faulty production of hemoglobin, which leads to the accumulations of red and purple pigments called porphyrins.” Decrease the level of porphyrins via some draining and things improve. Things like “sudden onset of bizarre behavior and strange outbursts.” Sounds like just the sort of behavior that would suggest bleeding, don’t you think?
Anyway, other diseases mitigated by bloodletting include a form of diabetes, hepatitis C (interferon treatment is more effective when you’ve got less blood, it seems), hemochromatosis, and polycythemia.
Not that a few rare diseases explain the notion that bleeding was considered an appropriate treatment for just about everything.