Is it just me, or does this sound kinda dangerous? That is, engineering a virus specifically to evade the immune system. As long as it stays un-mutated, fine. But what if a mutation occurs that results in it becoming a deadly disease, with the bonus of being "camouflaged from the immune system"?
It seems dangerous to me as well, but I should point out that viruses don't typically do that - they are either dangerous or not. They might change how infectious, or which species they target, but it rare to go from harmless to dangerous.
It's also not in their interest to be dangerous, it's better for them to simply be infectious without causing serious illness.
Unfortunately viruses (and genes for that matter) don't mutate intentionally for their best interests.
https://en.wikipedia.org/wiki/Talimogene_laherparepvec
https://en.wikipedia.org/wiki/Herpesviridae#Immune_system_ev...
They altered it to make it less virulent to normal cells and more lethal to cancer cells.
Considering that what you said can be translated to "we are curing cancer with an infectious disease that was modified to spread more slowly and kill cancer cells faster than it kills you", I don't think it is reassuring to the people that has to live day to day with the patient either.
It is a horrible necessity to have cancer patients be dosed with poison, but infecting themselves with something that can get to their families is in another league.
Dr. House is full of truisms and one pops up here. Doctors are also seriously concerned about the potential dangers of this treatment; however, they have determined that against the risks of cancer it's worth it.
No medicine is risk-free. Medicine replaces an active condition with a chance for a less severe condition. Cold sores (if they even present as a side effect) have a much better prognosis than cancer.