Fujin as a ghost originated from the medieval superstition that the flow of the air not only damaged agricultural products and fishing, but also got into the human body and caused disease.
妖怪としては、空気の流動が農作物や漁業への被害をあたえる、人間の体内に入って病気の原因となるという、中世の信仰から生まれたもの。