ABSTRACT
Although viral infections of the central nervous system (CNS) occur infrequently these infections are tremendously important given the potential neurological damage they can cause. As the highly specialized brain tissue is exquisitely sensitive to metabolic changes and that injured brain tissue recovers slowly and often incompletely, the CNS has inherent mechanisms for restraining immune responses. The CNS represents a very appealing site for persistent viral infections. First, many CNS cells are long-lived therefore allowing pathogens to niche or to survive within the same host cells for extended period of times. Second, the immune-privileged character of the CNS, such as low basal expression of major histocompatibility complex (MHC) molecules and limited migration of immunocytes, creates a unique environment that restricts the ability of the immune system to perform its functions (1). Nevertheless, the immune system can successfully clear viruses from the CNS without causing permanent neurologic damage. Multiple RNA and DNA viruses have the capacity to infect the CNS (Tables 1 and 2). The mechanisms used by some of these viruses to reach the CNS will be reviewed with a specific emphasis on the interactions with the blood-brain barrier (BBB).
