The term “Magic Bullets” is a translation of the German word “Zauberkugeln”. It is believed that Ehrlich took that term from the first German opera “Der Freischütz” by Carl Maria von Weber. In that opera a young man needed to shoot the target successfully to be able to marry his bride. To achieve this he made a pact with the devil and got a magic bullet that would automatically hit the target.
That found its way to the target like Ehrlich’s compounds were planned to.