A composition for treating cancerous cells in a subject having an immune system includes a virus in the Yatapoxvirus genus having at least one mutation. In one embodiment, the mutation results in suppressed expression of a TNF binding protein by the virus. In another embodiment, the mutation results in suppressed expression of thymidine kinase (“TK”) by the virus. In another embodiment, the mutation arms the virus with a transgene to express a bacterial flagellin. The mutations can be present singly or in combination. Additional aspects include a method of treating cancerous cells with a composition as described herein, and a method of delivering at least one gene to cancerous cells in a subject.