Stress
Ooh, yeah. You better believe it. Life and death hang in the balance on a daily basis, and you're right in the middle of it. Make one little slip and it could be the last of someone's days. The doctors get most of the credit for doing the healing and life-saving around the hospital, but both of you can be blamed (and certainly have trouble sleeping at night) if you make a fatal error.
Of course, most of the time, you're not going to accidentally kill someone who came in with a stomach flu, but there's no putting anything past you.