Is this done? Do you have any information on it working?

And how do you stop the virus from infecting healthy cells and causing more harm than good?

They way we do it now is as follows:

1. hospital sends blood of the cancer patient to us.
2. we filter out all the cells other than the killer T cells
3. we inject a virus that contains the genetic material that fights the cancer. The virus infects the killer T-cells and inserts its own genetic material into the T-cells.
4. we incubate and grow the T-cells a little over a week until there are enough to meet the required dose.
5. we wash out the viruses and cell food juices, freeze the cells, and send them back to the hospital.
6. doctor injects the cells back into the patient and they usually become cancer free within two weeks.

I don't see how you would change all the cancer cells in this way. It's more complicated people might think.