Don’t disregard “undiagnosed heart condition” so easily. I have a brother-in-law who was a healthy and very active person, passed the entry physical for the USAF and during basic training in Texas he collapsed during a run, and he was quite use to running and a skinny guy. He was taken to the hospital, where they discovered he had a heart problem that just decided to act up. He was medically discharged from the USAF a couple of months later.
I’ve also had experiences with young, physically fit lieutenants in a couple of different units who also died from undiagnosed heart problems. And one of them was a West Point graduate, thus in very good physical condition.
Hypertrophic cardiomyopathy, for example.