My mother was working for a (now extinct) biotech company that was developing a cancer 'vaccine' using a retrovirus to do targeted removal of cancer cells. One of the big problems they ran into was that the patient's immune system had a tendency to kill off all of their cancer-killer viruses.

So for a while there, they were seriously working on making a virus that would be completely invisible to your immune system- and this was maybe eight years ago. I'm sure there are dozens of little biotech companies in the bay area alone trying to do the same thing now.