If the Boomer pukes on you, or explodes on you, you're blinded and a generous helping of zombies will converge on you. The game doesn't really explain why the zombies don't come if you explode the Boomer twenty feet away. I'm not sure how the smell of Boomer bile on the ground beside you differs from the smell of Boomer bile on you.
Apparently the undead have a VERY precise sense of smell.
Shamus Young is a programmer and writer by trade, videogame nitpicker by inclination. If you have the patience for more of his ramblings, they can be found at ShamusYoung.com.