[true_religion]

Yes, the main problem with leeches in historical use was that medical theory believed disease was caused by a miasma and that drawing bad blood would cure the cause.

Today, we still use leeches but only for those cases where they can provably affect the cause.

[dekhn]

that's not really correct. we have a narrative for why leeches work, but it's not really a "provably affect the cause" sort of thing. We just know they eat dead tissue and suppress immune responses.