Now scientists have tweaked these viruses to avoid detection by the immune system, allowing them to track down cancer even after it's spread through the body.

On paper oncolytic viruses sound like a great treatment. These organisms are finely tuned to only infect cancer cells, leaving healthy cells alone – thus avoiding one of the main complications of radiation and chemotherapy.

But of course, things are rarely that straightforward. The human immune system is a well-oiled machine that filters out foreign invaders like viruses, regardless of whether they're here to help us. The problem can be worse than just wiping out the viruses before they kill off the tumor – in the worst case scenario, the immune system can go into overdrive, resulting in a potentially fatal condition called a cytokine storm.

For the new study, researchers from Emory University and Case Western Reserve have re-engineered an oncolytic virus to allow it to avoid detection by the immune system. This "stealth bomber" approach could be deployed throughout the body to give the therapy a new advantage – hunting down metastatic cancers.