Demyelinating central nervous system (CNS) disorders are a diverse group of conditions characterised by damage to the myelin sheath. These include not only primary autoimmune disorders such as multiple sclerosis (MS) or neuromyelitis optica spectrum disorder (NMOSD), but secondary demyelinating conditions caused by infection and neoplasm, where immunosuppressive therapy may worsen the condition or delay definitive treatment. We describe a young man with an unusual presentation of CNS demyelinating disease associated with HIV infection and positive syphilis serology. MRI brain and spine showed a demyelinating tumefactive lesion accompanied by longitudinal extensive transverse myelitis, and we initially suspected NMOSD. However anti-aquaporin 4 antibodies were negative, going against a diagnosis of NMOSD and he then tested positive for HIV which led us to consider TB myelitis, neurosyphilis and HIV vacuolar myelopathy. He was commenced on highly active retroviral therapy and treated with steroids and immunosuppression. He did not respond to treatment as expected so a brain biopsy was required to narrow the differential. Brain biopsy initially raised the possibility of progressive multifocal leukoencephalopathy which is associated with infection with the John Cunningham (JC) virus. Ultimately JC Virus PCR on the biopsy was negative, the final report suggesting nonspecific active chronic inflammation. We detail his clinical course and the diagnostic challenges along the way.