About 2-5% of AIDS patients develop PML. It is unclear why PML occurs more frequently in AIDS than in other immunosuppressive conditions; some research suggests that the effects of HIV on brain tissue, or on JCV itself, make JCV more likely to become active in the brain and increase its damaging inflammatory effects (Berger, 2003).

There are case reports of PML being caused by pharmacological agents, although there is some speculation this could be due in part to the existing impaired immune response or ‘pharm combos’ rather than individual drugs. Rituxan and Natalizumab (branded Tysabri).