Computer illustration of the hematoencephalic barrier between the blood flow and the central nervous system. It regulates the environment (homeostasis) in the brain by separating it from the blood. But even though the endothelial cells that line the capillaries are essential components of this barrier, the astrocytes surrounding the blood capillaries in the brain (pink) are also part of the hematoencephalic barrier. The blood- brain barrier stops harmful chemicals and bacteria from reaching the brain. Special drugs treating Parkinson's disease, Alzheimer's disease, schizophrenia and tumours may pass the brain-barrier. A new method involves the use of harmless viruses passing the barrier delivering treatment throughout the brain.