A curse is a bad illness where the one who curses and the one who has been cursed will both divide the “spoils” and both will receive their share of the proceeds. For, if it is an empty curse for no reason it will return to its place and sometimes even if there is a good reason for the curse, it will come back to haunt the one who cursed. Even if the curses are fulfilled, the one who cursed will be punished as he causes his friend to be punished and bad will come through him. Sometimes even curses which come from the mouth of a silly unrefined person will be fulfilled, as Chazal say “The curse of a simple man should never be light in your eyes”, and everything depends on the time and according to the minute will depend if the blessing or curse are fulfilled.
It is known that one of the three hundred and sixty-five negative commandments is that one should never curse any member of Klal Yisrael, and if he curses someone behind his back he transgresses “You shall not curse a deaf person!”. Since Chazal said that the guest who blesses the host will himself be blessed as it says “And I will bless those who bless you!”, one can extrapolate from here that one who curses a member of Klal Yisrael will himself be cursed as it says “And I will curse those who curse you!”. It follows that a person should always be careful that only blessings for other people should come out of his mouth and then he himself will be blessed.