In one aspect, the invention provides methods for preferentially killing target proliferating cells in a host, such as cancer cells, by infecting host tissues with two or more strains of virus. The strains of virus may be selected to provide a synergistic and symbiotic effect, involving a contemporaneous lytic infection in the target proliferating cells. In selected embodiments, the viruses are selected so that expression of a first virulence factor in proliferating cells infected with the first virus increases the lytic effect of the second virus； and, expression of the second virulence factor in proliferating cells infected with the second virus increases the lytic effect of the first virus. The genomes of the first and second viruses may be selected so that they are incompatible for recombination between the viral genomes in cells of the host.
