Why did people use leeches in medicine?

For centuries, many doctors worked with the idea that illness came from imbalanced humors. Within that framework, bloodletting seemed reasonable rather than bizarre.

Leeches were useful because they could remove blood in a controlled way, and their saliva contains compounds that prevent clotting. That made them practical long before modern medicine had better options.

Today, leeches still have a small place in some reconstructive procedures. That makes them one of history’s strangest medical tools to survive into the present.