<P> Late - stage neurological trypanosomiasis, or sleeping sickness, is a condition in which trypanosoma protozoa are found in brain tissue . It is not yet known how the parasites infect the brain from the blood, but it is suspected that they cross through the choroid plexus, a circumventricular organ . </P> <P> Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system that is caused by reactivation of a latent papovavirus (the JC polyomavirus) infection, that can cross the BBB . It affects immune - compromised patients and it is usually seen with patients suffering from AIDS . </P> <P> De Vivo disease (also known as GLUT1 deficiency syndrome) is a rare condition caused by inadequate transportation of the sugar glucose across the blood--brain barrier, resulting in developmental delays and other neurological problems . Genetic defects in glucose transporter type 1 (GLUT1) appears to be the primary cause of De Vivo disease . </P> <P> Some evidence indicates that disruption of the blood--brain barrier in Alzheimer's disease patients allows blood plasma containing amyloid beta (Aβ) to enter the brain where the Aβ adheres preferentially to the surface of astrocytes . These findings have led to the hypotheses that (1) breakdown of the blood--brain barrier allows access of neuron - binding autoantibodies and soluble exogenous Aβ42 to brain neurons and (2) binding of these autoantibodies to neurons triggers and / or facilitates the internalization and accumulation of cell surface - bound Aβ42 in vulnerable neurons through their natural tendency to clear surface - bound autoantibodies via endocytosis . Eventually the astrocyte is overwhelmed, dies, ruptures, and disintegrates, leaving behind the insoluble Aβ42 plaque . Thus, in some patients, Alzheimer's disease may be caused (or more likely, aggravated) by a breakdown in the blood--brain barrier . </P>

The is formed by a tight network of capillaries that protect the delicate tissue of the brain