The development of engineered viruses to selectively attack cancer has been underway for a number of years, and here is an example of an early trial for one strand of this research: "Scientists modified the vaccinia virus, which is more famous for being used to develop a smallpox vaccine. The virus, named JX-594, is dependent upon a chemical pathway, common in some cancers, in order to replicate. It was injected at different doses into the blood of 23 patients with cancers which had spread to multiple organs in the body. In the eight patients receiving the highest dose, seven had the virus replicating in their tumours, but not in healthy tissue. ... 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. Intravenous delivery is crucial for cancer treatment because it allows us to target tumours throughout the body as opposed to just those that we can directly inject. ... Infection prevented further tumour growth in six patients for a time. However, the virus did not cure cancer. Patients were given only one dose of the virus as the trial was designed to test the safety of the virus. It is thought that the virus could be used to deliver treatments directly to cancerous cells in high concentrations. ... Viruses that multiply in just tumour cells - avoiding healthy cells - are showing real promise as a new biological approach to target hard-to-treat cancers."