The 'mother' of the vampires (who turn out to be aliens from Saturnyne) explains the process of what they do to the young women.
This is how it works.
First we drink your blood till you're dry
then we fill you with our blood
It rages through you like a fire, changing you,
until one morning you awake and your humanity is a dream now faded
It's a gradual process that happens almost without being noticed. One of the girls, Isabella has been treated this way for a while now. While she is not yet fully transformed, she is not exactly her former self either - and she has little memory of what has caused this change.
For me, the ongoing battle between the good and not-so-good elements of life can be similar. Despite trying to be the best person i can be, there do seem to be other forces at play. Feelings of selfishness, resentment and other undesirable stuff. They sometimes do rage, and cause changes that erode away some humanity.
Fortunately, Amy managed to escape before any lasting damage could be done. I wish the same could be said for me.