Biology nitpick time! Yay!
Living virus is a bit of an oxymoron. Bacteria more likely.
This one's been laid to rest for some time now. Nowadays, no biologist in his right mind gives any credence to the idea that vriuses (virii?) are not living organisms.
I'd honestly suggest bacteria rather then viruses - it'd make sense for them to get to the brain. Viruses just...exist and replicate.
Bacteria have a much harder time getting through the blood-brain barrier, than viruses (virii?). And when they do reach the central nervous system, they tend to kill the host pretty quick, while viruses (virii?) may go for a acute, devastating infection (e.g. West Nile encephalitis) or a slow, dementia-inducing chronic infection (e.g. HIV and the "AIDS dementia complex").