Abstract: The present invention relates to chimeric viruses, the replication of which is regulated by a transactivation signal produced by diseased host cells. The chimeric viruses of the invention can infect both normal and diseased host cells. However, the chimeric virus replicates efficiently in and kills diseased host cells that produce the transactivation signal. The use of such chimeric viruses to treat infectious diseases and cancers are described. A particularly useful embodiment involves the modification of a murine parvovirus that infects human T cells to generate a chimeric parvovirus that is cytocidal to human T cells that express HIV-tat. The chimeric parvovirus can be used to treat HIV-infection.