What does the man in the car have the concerns Bud?

The human blood in the back seat concerns Bud.

If he would've let go of my arm for just one second I would've run like the devil was chasing me. On the side of the box some big red letters said as clear as anything, URGENT: CONTAINS HUMAN BLOOD!!!
Oh, man, here we go again! My heart started jumping around in my stomach. The only kind of people who would carry human blood around in a car were vampires! They must drink it if they were taking a long trip and couldn't find any people to get blood from.This guy figured he'd rather have my fresh blood than blood out of a bottle!