But not all viruses can pass thru "blood brain Barrier" in our body.

They have found out from autopsies only certain viruses can invade our neurons very successfully and they can lay dormant for many years. What I have read so far is that about 94% of people in USA have herpes of some type. I will post some articles on STDs, which is more prevalent and can cause neurological damage w/o any serious symptoms in adults, but in kids they are deadly to their brain cells in utero.
There are a large number of viruses that are out there naturally, which can attack brain cells than vaccinia viruses. That is my point here. I am very worried about Small pox viral attack now. Bioterrorism is more real than you ever know.