"That is a good image," [Van Helsing:] said. "Well, I shall tell you. My thesis is this: I want you to believe."
"To believe what?"
"To believe in things you cannot. Let me illustrate. I heard once of an American who so defined faith: 'that faculty which enables us to believe that which we know to be untrue.' For one, I follow that man. He meant that we shall have an open mind, and not let a little bit of truth check the rush of a big truth, like a small rock does a railway truck. We get the small truth first. Good! We keep him and we value him; but all the same we must not let him think himself all the truth in the universe."
No grisly account of silvery sharp fangs or glowing blood-red eyes is as unsettling as what happens to the Jonathan Harkers or John Sewards, when everything you've ever held sacred is turned on its head, to the point it's easier to believe in your own madness than to accept the madness of the world where such things could exist.
The ultimate classic vampire myth and a page-turner if I ever saw one.