Sci-fi is full of examples of engineered diseases that spread as viruses and usually they are so powerful that they kill almost everyone, and the possible cure is always the slowest possible, vaccinating people one by one.
Are there any real-life examples where engineered viruses are used for good, as vaccines for a known disease or for some other good purpose?
If not are there any scientific reasons why that wouldn't be possible?
In short, yes:
In the United States, the vaccine for polio is given as an injection that carries dead poliovirus. But there’s another form of the vaccine that is taken by mouth and uses a weakened—but live—version of the virus. It turns out that this version can briefly spread to other people before dying out. The World Health Organization has relied on the oral polio vaccine for its efforts to wipe out the disease worldwide. But there is a drawback. Rarely, the live vaccine can mutate enough to revert back to its virulent form. The oral polio vaccine carries three strains of the virus, one of which has been eradicated in the wild but is also most likely to cause this problem. The WHO is switching to a vaccine that has only the two safer strains. http://www.popsci.com/contagious-vaccine-virus
The idea behind a vaccine, is to make a mutated, weak, virus that mimics the strong one. This way your body can learn to defend against strong viruses, without the risk of dying.
When a virus reproduces, it has a chance (albeit slight) to mutate. The idea of contagious vaccines, while good in nature, requires the reproduction of a weak virus. This virus can therefore mutate, and become stronger. Either creating the original virus (see above), or a superbug. (this is already a problem with normal vaccines: http://www.who.int/mediacentre/news/releases/2017/bacteria-antibiotics-needed/en/)
So, while contagious vaccines have the potential to eradicate entire diseases, they also have the potential to spread the disease further, and even nullify the effects of normal vaccines.
You can, if you're in a hurry and have a source on hand, use the reasonably contagious, but pretty harmless, cowpox for mass infection immunisations against smallpox. In fact the standard vaccine for smallpox was for a very long time to infect individuals with cowpox and let it run its course. You can however just cram a bunch of people into a room with someone in the infectious phase of cowpox and get everyone sick at once.
