Passage: The blood–brain barrier (BBB) is a part of the capillary system that prevents the entry of T cells into the central nervous system. It may become permeable to these types of cells secondary to an infection by a virus or bacteria. After it repairs itself, typically once the infection has cleared, T cells may remain trapped inside the brain.  Gadolinium cannot cross a normal BBB and, therefore, gadolinium-enhanced MRI is used to show BBB breakdowns.

Question: What may cause the blood-brain barrier to be breached, trapping T cells inside the brain?
infection