This may be a bit TMI for you folks, but I’ll say it anyway :) I’m a pro puker, a victorious vomiter. When I was growing up I had some kind of weird hormonal issue which cased me to sporadically and suddenly throw up after feeling sick for 5–10 seconds before hand. You can see how it became necessary to communicate the following things to those around me quickly: one, I’m about to be sick, and two, I don’t need your help, I’ll be fine in a minute. And then, of course, to do the deed as discretely and neatly as one can do so. I have puking my guts out down to a fine science.
Nowadays the hormonal thing is gone (yay!) so I only get sick when I drink way too much or am ill. I do still announce it, mainly to discourage people from coming and helping me, because who really needs an audience while throwing up!
The crudeness of my language changes depending on who I’m with. “I’m going to be sick, I’ll be back in a second” is reserved for casual acquaintances, while the more colorful “Time for the technicolour yawn!” is left for the closest of friends :)