If a boomer has vomited on you, you will be quickly surrounded by infected zombies all trying to get a piece of you. The best way to defend this is to get everyone in a corner and melay. You can shed the vomit if you are playing online campaign with other people. If you are the only real person playing you can't do this. To rid yourself of the boomer’s vomit just click the start button and select the “take a break” option. You can then immediately hit the right trigger to retake control of your character. This is great if you can do it quickly to prevent the zombies from finding you.