Six years ago my sister ended up getting a stem cell transplant at 37 due to relapsed leukemia. For approx the first month she was in-patient in a very sterile environment and it was touch-and-go for some time. She then spent a few months in a quiet house away from the public, most friends, etc, slowly re-acclimating as she got all her immunizations back on board again and just got stronger little by little. Probably over 18 months after the transplant, she brought her kids out to the barn one day to see me ride. She was in the fairly clean barn for maybe a total of 30-40 min, not in a stall etc. Within a week she had been diagnosed with a fungal pneumonia out of nowhere. No one would ever blame the barn out loud, but the timing was too suspicious and I felt awful about it.
Another anecdote that is possibly unrelated but speaks to @VolteSomeday’s post: last winter I was diagnosed with a lung lesion/nodule with no prior history of any lung issues but with a long relationship with immune-related problems (catching viruses easily, prone to inflammation, not healing well). My care team ended up not being able to pinpoint the cause even after biopsy (we still watch it carefully), but the infectious disease specialist I saw was convinced the origin was fungal/something uncommon due to my crappy immune system and being in a barn around all manner of wildlife.
ETA: Before my sister’s experience, I never even knew anyone who had been through a transplant. I think two of the the things that make it so very difficult on your immune system are that 1) you are essentially zeroing out your old immune system so that your new one can take root and 2) you are very often having to use steroids to save yourself from graft-vs-host disease, which is a whole other bucket of dangerous worms. You are just walking such a fine line in that first year.