construction of a recombinant bacterial host which is substantially immune to a normally infectious viral agent is disclosed . in one embodiment a foreign recombinant reproducible gene segment is inserted into the bacterial cell . the gene segment is a partially defective variant of a gene segment from the infectious virus of interest . the viral gene segment , in its naturally occurring and fully operational form , normally codes for a non - repressor protein which participates in a multi - protein complex used in the development of the virus . however , the partially defective gene present in the bacterial cell codes for a defective protein that will bind to the remainder of the multi - protein complex with a greater affinity than the virus &# 39 ; fully operational protein . the &# 34 ; new &# 34 ; protein complex will therefore be inactive and will inhibit the ability of the foreign agent to use the multi - protein complex for its development .