The so-called July Curse is actually just a problem that many secluded islands possess.
Infant mortality was high, congenital deformities were common, including halted mental development, plus one couldn't ignore the unsanitary conditions and the overly simple and natural wild growing environment; all of these factors could lead to what is called the July Curse.
However, this notion that made outsiders deeply anxious had actually barely occurred over the past ten or twenty years.
With the island's opening, the influx of outsiders, and improved sanitary conditions, such issues basically didn't exist anymore.
As for the girl working in the kitchen, she did happen to be born in July, but that was purely a coincidence.
