[keefle]

Can someone explain the meaning of this to me?

Is the encounter with the vampire similar to forgetting why one came into a room because when encountering a vampire in a newly entered room you forget everything before that and focus on the vampire?

[zdbrandon]

The story is implying that whenever this happens to you -- forgetting why you came into a room -- the truth is that you were attacked by a vampire that made you forget the reason you were there, as well as the attack itself.