It is caused by a virus called JC virus which occurs in 70% of the population in latent form, causing disease only when the immune system has been severely weakened, as is the case for AIDS patients.
Ia disebabkan oleh virus JC, yang 70% populasinya terdapat di tubuh manusia dalam kondisi laten, dan menyebabkan penyakit hanya ketika sistem kekebalan sangat lemah, sebagaimana yang terjadi pada pasien AIDS.