Then you really need to consider that it's probably not as simple as "she has x disease and needs the
right antibiotic."
My best understanding: parasitic infections tend to thrive due to feeding on detritus from other infections and persistent infection in spite of multiple treatments probably means she's being reinfected on a regular basis.
My chronic infections did not begin to improve until I informed my then spouse "You are never touching me again without a condom because I think you are reinfecting me." following a conversation with a friend who was a physician. He had minored in parasitology and I believed I had an undiagnosed parasitic infection. He said it was plausible, given what I was going through.
That was a major turning point in my health. After nearly a year of being at death's door, the very next round of antibiotics stabilized me and I began gradually getting better.
Prior to that, I went through two periods a few months apart of needing new antibiotics every single time I ran out for 10 or 12 weeks. Nothing was really working.
The months in between? I was bedridden, sleeping 18 to 20 hours a day and probably having a lot less sex than usual. (But I honestly just don't remember how often I was having sex during that awful, awful time. I mostly remember hallucinating conversations with The Grim Reaper.)
(Disclaimer: This is not medical advice. I am not a doctor. I don't play one on TV.)