Or perhaps it's because a long time ago when I worked in Panama... I was bitten in my sleep by a bat.
7 seconds sound clip from the The Last Man on Earth (1964) movie soundboard.
You can hear this line at 01:09:14 in the Blu-ray version of the movie.
Quote context
[...]
- If I could find him and destroy him...
- But you said he was your friend.
- When I find him I'll drive a stake through him, just like all the others.
- But you lived through all this. Do you know why?
- My theory is that the bat had previously acquired the vampire germ.
- By the time it entered my blood, it had been strained and weakened by the bat's system.
- As a result, I have immunity.
- It's only a guess, but it's all I have to go on.
- You don't think that I'm immune, do you?
[...]