Last year, scientist used an engineered version of HIV to cure a child of leukemia. The procedure took months to perform all of the necessary steps, if not years, but in the end an engineered version of HIV successfully wiped out leukemia in a cld by specifically targeting the leukemia cells with the modified version of HIV to limit the growth and expansion of the cancerous cells.
This opens up a whole variety of new ideas involving Viruses and Medicine, specifically human modification. The Virus known as HIV has an extremely interesting set of properties, such as an ability to hide almost perfectly from the human immune system as well as a long term incubation. With these two properties, there could be a variety of things you could make to solve problems prevalent in the medical field today.
However, an even more interesting idea for the use of viruses in medicine is to set up a two step process for treatment, specifically you could make a virus much like HIV that could gently infect the cells of your immune system, never actually leaving the incubation period. However, the initial HIV cells injected into the host body would not actually generate more versions of the virus initially injected, but would instead produce a variant of initial virus which could be lethal to parasites, bacteria, virally infected cells, or even cancerous cells!
Let me try to explain this idea a little bit better. You could make a virus to infect your own cells, and add a string of DNA to synthesize another virus that could be coded to kill pathogens. By constantly producing low levels of virus lethal to certain pathogens, you could give your self immunity or even high resistance to pathogens!
Perhaps you could produce a hundred versions of the virus each coded to infect T-Cells, with each version of the virus aimed to constantly produce low levels of virus lethal to certain pathogens. You could grant a person immunity to a large variety of diseases! Imagine if you could