Post-herpetic Neuralgia
Research by Lev Neymotin
From the essay
This should be treated as a high-priority shingles/VZV complication until proven otherwise, while also ruling out other nerve, spine, medication, and cancer-related causes. The usual medical issue would be herpes zoster, caused by reactivation of varicella-zoster virus, sometimes followed by post-herpetic neuralgia. CDC describes shingles as a painful rash illness caused by VZV, and people with weakened immune systems are more likely to have atypical, severe, generalized, or complicated cases.
Adapted from PDF pages 1.