A new engineered virus has shown that it can selectively target and destroy cancer cells in what researchers call a medical first.

The findings, were published in the journal Nature, showed that the genetically engineered virus only attacked cancer cells and left healthy tissue alone.

"We are very excited because this is the first time in medical history that a viral therapy has been shown to consistently and selectively replicate in cancer tissue after intravenous infusion in humans," said Professor John Bell, lead researcher from the University of Ottawa.

"Intravenous delivery is crucial for cancer treatment because it allows us to target tumors throughout the body as opposed to just those that we can directly inject."

Scientists have been modifying viruses to attack cancer cells. Researchers have previously transformed the vaccinia virus to develop the smallpox vaccine. The new virus, named JX-594, is a breakthrough because it doesn't have to be injected directly into tumors to avoid the immune system.

The modified virus was injected in 23 patients with cancers. In the eight patients who received the highest dose, seven had the virus replicating in the tumors while completely avoiding the healthy tissue.

While the virus did not cure the cancer, it did prevent further tumor growth in six patients for a time. Researchers only gave patients one dose to test the virus. Scientists eventually hope that the man-made virus will deliver high doses of cancer-fighting drugs to the tumors, which will let the drugs destroy the intended cells without ravaging the rest of the patient's body.

"I believe that someday, viruses and other biological therapies could truly transform our approach for treating cancer," said Bell.