I believe every MedFet has it in themselves by nature. The MedFet community here have discovered it or are in the process of learning, although there are likely many out there who have it and do not recognize it, and they may never do so. Situation and timing is critical, most MedFets were in the right place at the right time to make the discovery.
One of the first things I ask a new MedFet contact is where they first developed their interest. Generally most found it earlier in life however I have seen a number of exceptions where it developed later in mid-life. It could be debated that in the past, it was commonplace for rectal temperatures and enemas to be used both at home as well as in a medical setting, and that this may have caused more people to develop the fetish earlier in life than they might today. I would suggest that although it may have increased awareness of those things, instead of bringing out more MedFets, it may have just as easily turned them off for life if they were too young or ill at the time.
The root of their MedFet interest is usually within an event which normally would be trivial or sometimes even very traumatic, but it was a trigger which awakened their fetish by causing sexual arousal in some way. Typically most did not find the actual event which triggered their fetish to be arousing, often stress and anxiety precluded arousal at the time, and it was something they discovered in retrospect in memories of the event. From there they usually build a fantasy of reliving or continuing to find new experiences, which may be life long.
There are no rules or stereotypes for being a MedFet in any variation, it comes in all flavors. Some enjoy enemas and rectal temperatures, others to less invasive aspects such as auscultation and breast palpation, and some to extremes such as anesthesia, surgical or dental role plays. Some strive to find their arousal in real time meetings, some enjoy an online experience chatting with others, and some simply enjoy reading or writing stories of their own and those of other MedFet's experiences. All share the same basic trait, arousal by something within a medical scene.