It is hoped reoviruses can be used to treat cancer, but there were fears they would not work if the immune system could wipe them out. A study published in Science Translational Medicine showed the viruses could hide in the blood and reach their target. Experts said it was an important step in advancing cancer therapies. Reoviruses are normally harmless, but they can cause stomach upsets and colds in childhood. However, it seems they have the ability to infect and kill some cancerous cells while leaving the surrounding tissue unharmed. However, experiments on mice suggested the virus would not survive in the blood as the immune system would destroy it. It meant the virus would need to be injected directly into the tumour or be given with drugs to suppress the immune system. Stealth mode A study in 10 people at the University of Leeds and The Institute of Cancer Research, at the Royal Marsden Hospital, showed that the virus could escape the immune system by hiding in the blood. All the patients had advanced bowel cancer which had spread to the liver, and were injected with doses of the reovirus ahead of their scheduled surgery. The virus was detected in the tumour, but not the liver, meaning it was selectively targeting the cancer. In the blood, the virus was detected in blood cells, not the liquid blood plasma all the cells float in, meaning it was "hitchhiking", the researchers said. Prof Alan Melcher, from the University of Leeds, said the virus was "even cleverer" than previously thought. "By piggybacking on blood cells, the virus is managing to hide from the body's natural immune response and reach its target intact." He told the BBC he had "no doubt" the virus would be eventually used "in combination with chemotherapy". 'Important next step' Dr Kevin Harrington, from the Institute of Cancer Research, said: "Viral treatments like reovirus are showing real promise in patient trials. "This study gives us the very good news that it should be possible to deliver these treatments with a simple injection into the bloodstream." Why reoviruses affect only cancer cells is not entirely understood. Cancer cells behave very differently to healthy cells, which may make them more susceptible to infection. Doctors are already testing the virus in some trials in people, such as studies on head and neck cancer. Prof John Bell, from the University of Ottawa, has researched using genetically modified viruses to attack cancer cells. He said viruses could be "exquisitely selective" in targeting tumours, and that this latest study had shown how safe the technique was. "This study is an important next step in advancing oncolytic virus therapies into cancer patients."