This is potentially good news.

The U.S. National Cancer Institute has just decided to fund multiple human clinical studies to test the reovirus. This naturally occuring virus has a remarkable ability to infect and kill cancer cells, without affecting normal, healthy cells.

It appears that almost all of us have a natural immunity to this virus due to it being so pervasive in the environment.  However, a cancerous cell does not have the same immune response as a healthy cell, which means that the virus ends up killing the cancer cells and propagating itself to other cancerous cells.