Doctors do not "crack under stress." Not stress from their work, anyway.
Anybody who can handle 48 hours on, 24 hours off as an intern in a big city hospital for a year does not "crack under stress." On my busiest night, I by myself admitted 24 patients to the hospital from the ER. Only one died in the process that busy night, as I recall. An athletic 45 year-old diabetic dad with an MI. Some died shortly afterwards, though.
The worst night that I remember in the ER was a Saturday afternoon when two large families in small cars had a head-on in a parking lot. We had to separate the hopeless parents and kids from the others, fast. There were only two of us on duty. I think two or three kids and one parent survived.
Taking care of people with PTSD - if it even really exists as a "disease" - ain't nothin compared to that kind of work.
And that kind of medical work ain't nothin compared to combat.