Your comment is downvoted, but I had the exact same thought.
What are the long-term risks if we start injecting people with large quantities of a virus to battle another decease? Could that virus evolve into something much worse than cancer?
It's not the injecting large quantities of virus that poses a risk, it's genetically modifying viruses and then exposing people to them that is risky.
I would hope that quarantine procedures around any human made-or-modified virus would be quite strict to avoid something unwanted making it's out of the lab.
What are the long-term risks if we start injecting people with large quantities of a virus to battle another decease? Could that virus evolve into something much worse than cancer?