Supposedly, a vampire cannot come into a house unless invited; evil is much the same with respect to the human heart.
It seems likely that humans are intrinsically protected by a kind of natural innocence - but that this protection is breached if evil is invited.
This still applies, now as ever, but temptation in the modern world resembles having a lifelong series of vampires ringing the doorbell and requesting or demanding admission; some apparently beautiful, others viscerally sexy; some dominant and demanding, others dependant and desperate...
Sooner or later, most people will be worn-down or taken off-guard say yes; invite the vampire into their home - evil into their heart.
But once the vampire is inside it may be much more difficult to get rid of it.
Not least because you may be bitten and yourself become a vampire: one whose pleasure is to prey upon the innocent, to corrupt the Good.