You have probably heard mention of viral vectors in connection with gene therapy and biotech. Just what is a viral vector? Well, look at it this way. You have something you want to deliver to someone else. You can put the message in magnetic form on storage media, write it down and send through snail mail, alter a plasmid and use bacteria to send it ... You get the idea. You can also slip it inside a virus that has been rendered harmless and use that to deliver that something to someone else. Using a virus for this purpose is known as using a viral vector.
Viruses are usually associated in our minds with thing make us sick, e.g., COVID and the common cold. But fundamentally a virus is an organism that is incapable of replicating on its own. It has evolved to be able to carry its genetic material into healthy cells, where it then is replicated along with the cell.
Researchers wondered why they couldn't scoop out the harmful genetic material in a virus, slip in some beneficial DNA, and re-introduce the virus. All that involves some very interesting science, but the bottom line is that we now have the ability to "turn" a virus from an agent of doom to an agent of good.
Not all viruses fit the criteria. For one thing, it has to be a virus that a person hasn't already encountered. Why? Because if the person has developed an immune response to that virus, it will destroy the virus before it has a chance to insert the beneficial material. It also has to be a virus that we understand - thoroughly. Why? Because we're introducing it into the human body. If we don't understand the mechanism and the way it works in the human body we could do a lot of harm while trying to do a lot of good.
You can see more about them in the video from ASGCT below, but the bottom line is that:
Adenoviral vectors (AdV) were the first used in gene therapy. These viruses are the virus that causes the common cold. Since these AdVs caused some serous immune reactions in patients, they are now under study for future use.
Lentiviral vectors (LVV) are a species of retrovirus - using RNA rather than DNA - that inserts their genetic material in a number of ways. Because of this, they are mainly used outside the body with the modified cells being infused into the patient.
Adeno-Associated Viral (AAV) vectors have gained interest since about 2012. These viruses do not become a permanent part of the DNA receiving the vector. They also carry a small "load," making them good for some purposes but not for others.
For a deeper dive into viral vectors right now - bluebird bio's genehome | Gene Therapy Net | PubMed | NIH National Institute of Environmental Health Sciences