That infectious agent could be a type of polyoma virus.
From the newscientist.com
A small study in 2002 found a common polyoma virus, known as JCV, in the majority of 20 samples taken from brain tumours.
From the newscientist.com
Dr. Leon Goldman, a University of Cincinnati dermatologist, reports that doctors can say for certain only that warts are produced by a polyoma virus, a highly contagious carrier.