It occurs almost exclusively in people with severe immune deficiency, such as transplant patients on immunosuppressive medications,[1] patients receiving certain kinds of chemotherapy, patients receiving
natalizumab (
Tysabri)[2] for multiple sclerosis, psoriasis patients on long-term efalizumab (Raptiva)[3] or AIDS patients.
It is caused by a virus, the JC virus, which is normally present and kept under control by the immune system.