It doesn't have to. It does not always mean ONE set to failure either. It is measured by the amount of Work done per unit of time. If you work out for one hour, the more work that you do in that hour, than the higher the intensity. Don't get me wrong, this definitely does happen with me too, but it is not the GOAL or the way to always measure how hard someone has worked.
Some guys are just more prone to get sick while others never do.
Most of my people only get sick during the initial conditioning phase of training and once they are conditioned, they rarely do. The truth is, some of the hardest workers that I have ever seen have never vomited. It happens sometimes, yes, but it is not a way to measure a good workout. I start beginners with high reps of at least 10 or more per set. I believe that it is healthy both mentally and physically to change your rep range periodically.