A new study of a Taiwanese cohort found a high prevalence of resolved hepatitis B virus (HBV) infection in people with neuromyelitis optica spectrum disorder (NMOSD). Resolved HBV was found in 63.4% of people with NMOSD, compared with 16.4% in healthy controls, equating to a 5.7-fold increased risk of NMOSD following HBV infection. Furthermore, resolved HBV was associated with later onset of NMOSD and higher disability scores compared with individuals without HBV infection when adjusted for age and sex, suggesting that the immune response to HBV could contribute to NMOSD pathogenesis.