Used in research to battle: Blood clots

Bloodsucking leeches have been used for centuries by doctors practicing bloodletting to rid the body of disease. Frowned upon in modern medicine, it's now making a comeback.

Hirudin, a substance in their saliva, stops blood clots forming on the wound and actually promotes healing. Bivalirudin, an artificial form of this, cuts the risk of repeat heart attacks by a third.