This is a workaround for 2 Problems:
A) The Boomer can't vomit for about 2 seconds after spawn. He CAN however melee ( therefore SLAP!! ) a Survivor and bile him then.
B) Boomers Bile does not take effect immediatly. If you run around a corner and start biling, the Survivors still have about one second to melee you. Now if you slap them first they dont
If the Survivor melees you faster than you can melee him, he has won
NOTE:
Melee has a scythe-like huge arc and attack duration (seriously, WTF). If he started a melee attack and you run into it, Slapping wont stop it or save you.