techfun89 writes "Viruses can make us all sick, but one day could be engineered to defeat cancer. Cancer cells have one trait that may leave them open to attack. They aren't good at killing off viral infections, hence, at least in theory, you could use a virus to kill cancer cells without affecting the patient. Dr. Ian Mohr, a virologist at New York University, altered the herpes virus so that it isn't attacked by the immune system and kills cancer cells more efficiently. Another virus that is proving effective for liver cancer is Vaccinia. Vaccinia is used to protect against smallpox and so far the results have been promising. Several groups of patients have had an increase in survival times. Meanwhile other viruses are being used for things like melanoma, bladder cancer, and head and neck cancer."