The present invention is directed to the isolation and use of super-infective, tumor-specific vectors that are strains of parasites including, but not limited to bacteria, fungi and protists. In certain embodiments the parasites include, but are not limited to, the bacterium Salmonella spp., such as Salmonella typhimurium, the bacterium Mycobacterium avium and the protozoan Leishmania amazonensis. In other embodiments, the present invention is concerned with the isolation of super-infective, tumor-specific, suicide gene-containing strains of parasites for use in treatment of solid tumors.