If it killed only bad bacteria, it would be a cure for cancer. Cancer is a virus that attaches itself to cells in your body and then proceeds to destroy the cells. Some cancers, such as skin cancer, can be treated if diagnosed early. This happens simply by the virus-infected skin being amputated, which removes the cancerous cells. Others, such as pancreatic cancer, cannot be treated. You can't just remove someone's pancreas and expect it to be okay. They just have to stay on life support and chemotherapy. With your miracle version of bacteriophage,(not capitalized) which found some time ago, we would be able to cure any cancer, and it would be just as treatable as a common cold. Bacteriophage doesn't just kill bad bacteria, it kills all bacteria. This would theoretically cure cancer, but also kill the person, which we don't want to do. As you can see, the bacteriophage is technically a cure, but not ideal, as it kills the host of the bacteria as well.