Joined: Oct. 2005
I take it your "bad guys" don´t want to order a traceable whole-virus sequence mail-order...sensible, even if not just any molecular sleuth would be able to easily get past customer-synthesis lab confidentiality. Then they have to own the (DNA or RNA) solid phase synthesis equipment themselves. Not uncommon in research labs, but possibly traceable. Then there´s the question of high-security lab facilities; a "human" virus intended to target the brain doesn´t sound very "safe" to work with...who would take the risk?
Note that synthesizing viral nucleic acids from scratch is subject to some size constraints (smaller virus genome=easier in principle, large enough=not possible today). Protein synthesis can be made using cell-free extracts. Typically, viral coat proteins self-assemble into viral particles and incorporate the viral nucleic acid. Many (but not all) viruses should be relatively easy (famous last words, I know!) to produce in a cell-free system.
Perhaps a more interesting question implied in your post is, how does one custom-design such a viral genome to i) target the brain and ii) produce a predictable effect? Here´s a review article you might want to read. It describes tissue tropism - cell/organ targeting - within the viral "family" containing polio as well as common cold virus. Polio virus is interesting here, as it targets cells within the central nervous system (not brain cells, but anyway).
Hope that helps.
/The Rev (no, not that one)