I had my first major medical procedure probably within a day of birth: I was born with spina bifida and had to have my lesion closed. So, I don't remember how that all went. As far as my particular medfet (masks), I never got into wearing masks, but very early on I got to liking female patients in masks. At first, it was when a girl was being given anesthesia with a "black beauty" mask. I fixated on those kind of pics or scenes quite a bit in books or on TV.
Later on, maybe in my tweens, it evolved into what it primarily is now, which is patients in surgical/N-95/other protective masks. I remember seeing Entertainment Tonight, which was showing footage in a hospital. One of the girls, who was about my age, a tween, had on a white nightgown (think Clara from the Nutcracker), and was wearing a white, tie-on surgical mask. It got my heart racing.
I have never had to mask up for medical reasons, so I'm not sure why this particular medical fetish arose, versus, say, exams, urodynamics, surgical prep, etc. I DO have much experience in all that, dating from just about birth.