The invention provides gene constructs comprising a nucleotide sequence(s) encoding an antibody, antibodies, or part(s) thereof, that specifically bind to structural or non structural proteins of a pathogen(s). The invention also provides a nucleotide sequence(s) encoding a peptide, or a protein(s), or enzymes that are detrimental or toxic to the said pathogen(s). The invention provides the means and methods for the in vivo assembly of two or more proteins, encoded by separate nucleotide sequences, into a protein complex. These antibodies, toxins or complexes may be expressed and targeted to cellular membranes or plant cell compartments in different orientations and also can be cleaved in vivo by different proteases to become active. These agents are named 'molecular pathogenicides'. The means and methods are provided for immobilizing a recombinant protein capable of binding a pathogen, by recombinant DNA techniques. The means and methods are also provided for expressing molecular pathogenicides in membranes of monocotyledonous or dicotyledonous plants.