Difference between 防ぐ and 予防する


What is the difference between 防ぐ and 予防する ?

To me both of them seem to mean "to prevent".  
A Japanese person once explained to me how they are different but
unfortunately I don't remember.

Similar question: [What is the difference between 防止 and
予防?](https://japanese.stackexchange.com/questions/6436/what-is-the-difference-
between-%E9%98%B2%E6%AD%A2-and-%E4%BA%88%E9%98%B2)




予防する ＝ 予め防ぐ. 予 of 予防する means 予め（あらかじめ, in advance）. So 予防する is to prevent in
advance. Yes, the meaning of 防ぐ(prevent) itself includes the concept 'in
advance', but 予防する emphasizes the meaning.

我々は火の手が広まらないように防いだ.(We prevented the fire from spreading. ) In this case, we
don't say 予防した, because the fire actually happened already.

