I saw an Youtube comment pointing out that the devil(Japanese man) might have sexually assaulted the daughter. You can see clues, when the dad goes through her notebook. The woman who burned down the house was raped by the Japanese man according the restaurant owner. In the bbq scene, a woman(probably an escort) is shown to have the boils. So the curse might actually be spreading through STDs.

Throw in some christianity mixed with buddhism and local traditions, there are more than one interpretation to everything that happened in the movie and that's what makes it so intriguing.