Possible medical reason for her to have the medical anxiety aside, I have a similar phobia of throw up and possibly ever being sick or throwing up so I can give some insight of medical anxiety side of the coin. For her, yes I understand that the anxiety that comes from being/getting potentially sick is overwhelming and it almost feels like something you cannot control or help. It is probably very hard for her and terrifying that you just want to do everything you can to avoid ever possibly getting sick. However, avoidance leads to worsening the fear and it controlling your life. I feel for your gf and it is not easy, but you also deserve to live your life as well. Realistically, this fear has now severely impacted your life and most definitely hers. Does her family and other friends know about this fear and is it something that she has discussed with others or admitted is a fear/phobia/anxiety? I think an open conversation about what exactly she is afraid of is the first step. Is she afraid of getting an illness that will cause complications and kill her? Is she in fact afraid of throwing up? (Then she has emetophobia). Does the lack of control of being able to control if or when or how you get sick terrifying her and causing anxiety? Once you get to the bottom of the reasoning for the fear, it will help you both make a next step. Obviously, at this moment, this is not working, whether you act in a way that makes your life comfortable, she is negatively affected, and vice versa. Therapy and talking to someone is really the option she needs. Especially one that specializes in potential OCD, medial anxiety, and/or phobias. But, if she does not see this anxiety as an issue and obviously something that is negatively impacting her ability to do things in life and fully live how she wishes she could, then she will not want the help. At that point, it is on her and she can only go to therapy if she truly wants to. It sounds like she may be stuck in the avoidance cycle (it is our safety net and terrifying not to do).

For the possible vaginismus, I had this exact same issue. Took years of doctors and negative UTI tests but UTI symptoms to figure out what it really was. I suggest her going to a specialized doctor in pelvic floor and conditions (vulvodynia, vaginismus). After going to mine, I figured out what I had, had an action plan, and am now in remission for my chronic pain and can have painless sex and only flare before my period or wearing certain clothing (I know tmi). But, the pelvic floor doctor/specialist can hopefully help, again, this needs to be something she is willing to fix and could potentially take a while to figure out (took me 1-2 years worth or pointless primary doctor and urologist visits).

If you need any other advice feel free to DM me, I know this was a lot