I'm doing a paper on West Nile virus for my microbiology class. One of the things I'm supposed to address is mechanisms of pathogenesis, including why the virus is more likely to cause encephalitis in the elderly. I know that in order to cause this condition it has to cross the blood brain barrier and infect the brain parenchyma. Parenchyma is defined as: "The distinguishing or specific cells of a gland or organ, contained in and supported by the connective tissue framework, or stroma." More specifically, it infects neurons , especially in the deep nuclei and grey matter of the brain, brain stem and spinal cord. The main thing which help the virus pass the blood brain barrier are stimulation of a specific kind of "toll like receptors" which bring about an inflammatory response by signaling for cytokines and other non specific immune system tools. Specifically tumor necrosis factor alpha is the one which mediates increased BBB permeability. Here is an interesting article I found about a study which shows that mice deficient in a paticular toll like receptor experience reduced brain involvement during WNV infection: http://www.medscape.com/viewarticle/495916_3 Another article with some more explination: http://www.medscape.com/viewarticle/532913_3 Basically, what I've found is that this inflammatorry response is helpful in reducing viral replication in the periphery but has the horrible side effect of increasing BBB permeability. In addition, the inflammatorry response itself is also destructive to brain cells. The majority of damage is caused by direct viral infection of neurons. So ultimately the process does way more harm than good because if the virus stays mostly confined to the periphery our bodies almost always do a fine job of beating it, unless we are immunocompromised. Some questions: 1) Is meningitis a specific kind of encephalitis? I'm trying to make sure I understand the distinction between these two terms. Any explination of these two terms is appreciated. It is kind of overwhelming trying to read about this stuff because every definition I read contains at least 2 other words I need defined, like a never ending russian doll of medical terminology. 2) Does anybody have any insight into how this fits into why the Elderly experience a much higher incidence of encephalitis? I am speculating it has to do with their reduced immune systems or maybe that the permeability of the BBB increases with age? It may also have something to do with reduced levels of Tumor necrosis factor 3. Actual references would be awesome, but informed speculation is appreciated too. Edit: I should also add that children are less susceptable to encephalitis. This may offer another clue.