Viral trafficking

• Virus infections usually begin in peripheral tissues and can invade the mammalian nervous system (NS), spreading into the peripheral (PNS) and more rarely the central nervous systems (CNS).
• The CNS is protected from most virus infections by effective immune responses and multi-layer barriers.
• However, some viruses enter the NS with high efficiency via the bloodstream or by directly infecting nerves that innervate peripheral tissues, resulting in debilitating direct and immune-mediated pathology.

Most acute and persistent viral infections begin in the periphery, often at epithelial or endothelial cell surfaces.

Infection of cells at these sites usually induces a tissue-specific antiviral response that includes both a cell autonomous response and paracrine signaling from the infected cell to surrounding uninfected cells by secreted cytokines .

This local inflammatory response usually contains the infection. After several days, the adaptive immune response may be activated and the infection cleared by the action of infection-specific antibodies and T cells (which are  one of the major components of the adaptive immune system)

Viral infections that escape local control at the site of primary infection can spread to other tissues where they can cause more serious problems due to robust virus replication or, overreacting innate immune response.  Such a response in the brain is usually devastating and can lead to inflammation (swelling) of the protective membranes covering the brain and spinal cord (Meningitis ), inflammation of brain (meningoencephalitis or encephalitis) or death.