Did they cheat?

My girlfriend tested positive for syphilis and told me but if she didn't the nurses or doctor would have had to inform me. I was than tested and tested negative for it. My girlfriend says that she apparently caught it and had it a year before we were together and that's why I never got it because it wasn't and hadn't been contagious since we got together...
Is it possible that could be the truth or is she lying, which means she cheated?
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I read on your profile that you are in Healthcare. Is there anywhere I can phone or go to speak to a medical professional or something to get there opinion that way I can have zero doubtsI love her very much but if she betrayed me like that, well than I don't think I can be with her. Trust is a very big deal for me but she keeps denying it and I'm not educated on the subject to argue with her or even stop from doubting myself when she's tells me I'm wrong
Did they cheat?
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