A number of years back, there was a great deal of excitement about using viruses to target cancer. A number of viruses explode the cells that they’ve infected in order to spread to new ones. Engineering those viruses so that they could only grow in cancer cells would seem to provide a way of selectively killing these cells. And some preliminary tests were promising, showing massive tumors nearly disappearing. But the results were inconsistent, and there were complications. The immune system would respond to the virus, limiting our ability to use it more than once. And some of the tumor killing seemed to be the result of the immune system, rather than the virus.