More than a century ago, observations revealed that certain natural viral infections (e.g., West Nile virus and mumps virus) were associated with spontaneous cancer remissions . These viruses were subsequently shown to have a natural preference for cancer cells and infection with these oncolytic viruses (OVs) triggers lysis of infected cells as well as activation of anti-tumoral immunity . Advances in molecular biology have also allowed the modification of other viruses to make them specific to neoplastic tissues and/or to combined them with immune reagents to break tumor-induced immune tolerance . For instance, recombinant measles viruses have been used to treat human patients with bone-marrow cancer . Interestingly, this treatment only led to a significant resolution of tumor in two patients who were measles-seronegative. Recently, a genetically engineered virus called T-VEC virus has been approved by the US Food and Drug Administration to treat advanced melanoma .|
Several studies have also focused on biological anticancer agents based on oncolytic bacteria. In 2014, Roberts and colleagues tested the oncolytic potential of Clostridium novyi, a bacterium extremely sensitive to oxygen that permits the specific targeting of cancer cells, in the center of solid tumor, that are in a hypoxic environment . A derivative of the wild-type strain (C. novyi-NT) has been engineered to become inoffensive for the host  and tested via intratumoral injection against natural canine tumors as well as on advanced leiomyosarcoma in human patients . C. novyi-NT destroys cancer cells, but also induces a rapid and robust local antitumoral response. Such experiments pave the way for considering pathogens as new therapeutic opportunities to eradicate neoplastic tissues.