Speculation on my part: their chances of contracting a more virulent strain were increased by working in the hospital.
Bunch of doctors and nurses in China, young and in good health, have been killed by this. Presumably as medical personnel on the front lines, they got the best healthcare that could be provided.
I have been watching YouTube videos of an emergency nurse in Wuhan who has it, her husband is making close to daily videos of what she has been going through. She has had it for more than 3 weeks and is still in the hospital although she is slowly getting better.
Possibly.
Hopefully that was the case.