Stress
At first, maybe a little. But once you warm up to the idea of… doing what you do, this job is no more stressful than any other in which you work twelve hours a day and have to tell people they're dying.
Okay, so parts of the job are a bit rough. The idea, however, is to become a machine. A machine that relates to its patients and makes them feel safe and cared for, of course, but a machine nonetheless. If you can (at least on the inside) look at curing the human body the same way a mechanic goes about fixing a broken alternator, you should be able to perform your duties without feeling like doody.