1. Infectious disease (preferably a virus)
2. Primarily strikes down people in their prime
3. Doesn't affect children as badly, if at all
4. Can affect the brain
I'm making *up* a disease for a science fiction themed game, but any info on existing diseases will help me out in creating a realistic illness. #3 is the case for the fake disease, but if I knew of real diseases like this, I might be able to come up with a quasi convincing explanation that wasn't Star Trek technobabble.