immune system to the tumour and may help to generate useful and long-lasting antitumor immunity. This essentially produces a personalised cancer vaccine. Many cases of spontaneous remission of cancer have been recorded. Though the cause is not fully understood, they are thought likely to be a result of a sudden immune response or infection. Efforts to induce this phenomenon have used cancer vaccines (derived from cancer cells or selected cancer antigens), or direct treatment with immune-stimulating factors on skin cancers. Some oncolytic viruses are very immunogenic and may by infection of the tumour, elicit an anti-tumor immune response, especially viruses delivering cytokines or other immune stimulating factors. Viruses selectively infect tumor cells because of their defective anti-viral response. Imlygic, an attenuated herpes simplex virus, has been genetically engineered to replicate preferentially within tumor cells and to generate antigens that elicit an immune response. Oncolytic behaviour of wild-type viruses Vaccinia virus Vaccinia virus (VACV) is arguably the most successful live biotherapeutic agent because of its critical role in the eradication of smallpox, one of the most deadly diseases in human history. Long before the smallpox eradication campaign was launched, VACV was exploited as a therapeutic agent for the treatment of cancer. In 1922, Levaditi and Nicolau reported that VACV was able to inhibit the growth of various tumors in mice and rats. This was the first demonstration of viral oncolysis in the laboratory. This virus was subsequently shown to selectively infect and destroy tumor cells with great potency, while sparing normal cells, both in cell cultures and in animal models. Since vaccinia virus has long been recognized as an ideal backbone for vaccines due to its potent antigen presentation capability, this combines well with its natural oncolytic activities as an oncolytic virus for cancer immunotherapy. Vesicular stomatitis virus Vesicular stomatitis virus (VSV) is a rhabdovirus, consisting of 5 genes encoded by a negative sense, single-stranded RNA genome. In nature, VSV infects insects as well as livestock, where it causes a relatively localized and non-fatal illness. The low pathogenicity of this virus is due in large part to its sensitivity to interferons, a class of proteins that are released into the tissues and bloodstream during infection. These molecules activate genetic anti-viral defence programs that protect cells from infection and prevent spread of the virus. However, in 2000, Stojdl, Lichty et al. demonstrated that defects in these pathways render cancer cells unresponsive to