Virus is a non-living but infectious structure consisting of a nucleic acid core and envelope covering. The envelope is what determines target specificity.
If you are genetically engineering, lets say a retrovirus (which if you are writing a sci-fi book this is probably the type of virus you are wanting to write about, for many reasons) you would want to use peices of DNA that code for integrated expression cassettes for viral Gag, Pol, and Env proteins, all of which are required in trans to make virus.
These DNA's would be mixed and then put into a living mammalian cell (what we cell biologists call a packaging cell). This packaging cell line transcribes and translates the 3 cassettes and builds the virus.
The gag gene encodes internal structural proteins, pol encodes reverse transcriptase
and integrase, and the env gene encodes the viral envelope protein, which
resides on the viral surface and facilitates infection of target cell by
direct interaction with cell type-specific receptors; thus the host range of the virus is determined by the design of the envelope proetin.
As for some example envelopes, the amphotropic envelope protein has historically been the protein of choice for infection of human and other mammalian cell lines. The 10A1 envelope protein has also been used due to its increased versatility relative to the amphotropic