I thought I would offer some of my experiences to this thread.
I have been diagnosed with ME CFS and pan-hypogammaglobulinemia, low IgG, IgM and IgA. I have been on both acaclovir and valciclovir as a prophylactic but have still had a constant rash on the neck which was originally diagnosed as shingles. I recently tried famicilovir for 10 days and the infection got worse.
My immunologist has eventually agreed to a biopsy to try and discover the infection. He thinks it could be HSV1 or shingles.
What I've discovered is that shingles (or what ever my persistent infection is) can become resistant to acaclovir, valciclovir and probably famicilovir. Possibly other infections it's designed for too?
I'm also on immunoglobulin therapy and this did show signs of controling the infection and I started to feel better with a little more energy after about 4 months. However I then caught an infection and 6 months later still haven't recovered. The obvious thing to conclude is that it was possibly a COVID 19 variant, but I never tested it.
I started a course of cimetidine 20 days ago, but this shows no signs of improvement in my infection and general health.
I've also been able to get hold of (or at least it should be in the post soon) 14 days of Zostex, but reading through the information on this, it's best administered at the beginning of the infection and not in my case after years of it being active.
I'm in the UK with the NHS and the wait for an appointment is approximately 6 months and then there will be a further wait for the biopsy. So I won't have any answers soon.