[colarris (OP)]

Is there such a thing as a 'good virus', one that infects us with positives results? If not can't we produce any? Could they not be produced to destroy more harmful ones?

[Bored chemist]

Getting cowpox will stop you getting smallpox.
Also, there are viruses which prey on bacteria. If you have a bacterial infection then, in principle, it can be treated with  a virus.
https://en.wikipedia.org/wiki/Phage_therapy

[evan_au]

It is thought that much of the "junk" DNA in our cells, adding up to about half of human DNA originated with "jumping genes" coming from viral infections.

Some of these jumping genes now play a part in essential developmental processes.

The details are still unclear but there have been suggestion that they play an active part in:
- The immune system
- Uniquely marking growing nerve cells in the brain so that cells connect to the correct targets, instead of themselves
- The placenta

See: https://en.wikipedia.org/wiki/Transposable_element

Or listen (1hour 10 minutes): https://soundcloud.com/7th-avenue-project/sofie-salama-radio-interview-the-story-of-jumping-genes-1

[colarris (OP)]

Many thanks for the replies.

[chris]

@colarris you are welcome.

To add to the points made above, there is evidence that prior infection with certain viruses may help to prime the immune system and establish a more powerful immune tone enabling an individual to better defend themselves against other types of infection (bacterial) subsequently.

The study to which I am referring was done by scientists in the US using two classes of herpes virus, one the mouse form of glandular fever, the other the mouse form of CMV (cytomegalovirus); animals infected several weeks previously with these agents were resistant to subsequent challenge by infection with Listeria monocytogenes (a bug that sometimes causes meningitis) and also Yersinia pestis (plague bacteria); mice that were not previously infected with either of the two herpes viruses all died following an identical challenge.

The team speculate that the ancient history of herpes, which goes back over 200 million years, means that we have in some ways evolved to depend upon infection with these agents for the normal maturation of the immune system (more than 90% of humans carry EBV and about 50% have CMV).

This is a transcript of an interview I did with Skip Virgin, the lead author of the study reported in Nature (Nature 447, 326–329) in 2007.

Chris Smith: ...fighting fire with fire, scientists have found that being infected with members of the herpes family of viruses seems to strengthen the immune system and that may make us better able to fight off bacterial infections such as Listeria or the plague. Here is 'Skip' Virgin.

Skip Virgin: We have been focusing on a particular relationship between an infected host and a set of viruses called herpes viruses. These viruses have the ability to go in and persist for the entire life of the host and our big picture question is how does that work for the viral genes that contribute to the ability of the virus to persist, despite the efforts of the host to remove the virus, and then what are the host genes that keep the virus under control during that lifelong infection that prevents the virus from emerging and causing disease over the life of the host, so that particular equilibrium or balance, and how that works is our major focus.

Chris Smith: And what have you actually described in this paper, what have you found?

Skip Virgin: We had a long-held view that fundamentally these herpes viruses are harmful and in the course of our studies of those mechanisms of how the equilibrium between the virus and the host are maintained in the mouse model that we used, we made a surprising observation, which is that there is actually a benefit to the host conferred by these viruses.

Chris Smith: So, when you say benefit, in what way are these animals actually getting some kind of benefit from being infected with a virus?

Skip Virgin: Well, the virus itself does them no clear harm and because the virus is present, they turn out then to be resistant to infection with two bacteria that we used, one is called Listeria monocytogenes and the other is Yersinia pestis, but the disease that it causes is the plague, or 'black death'.

Chris Smith: Now, those are the two bacterial infections which establish quite invasive infections quite quickly, how does the virus in the presence of the virus, even a long time after you get infected with it, actually trigger this robust immunity? What is the virus doing to achieve that?

Skip Virgin: Well, the virus is maintained in a host in the latent state. But intermittently it emerges from the latent state and begins to replicate and then the host recognizes that, and an immune response is stimulated and a hormone called interferon-gamma is released, and that hormone then activates cells that are responsible for defence against bacteria such as Listeria or plague. Those cells are now able to resist that challenge with the virulent bacteria.

Chris Smith: And you could see why there might be an evolutionary benefit here, because these viruses want to persist in that host for the lifetime of the host, so if they make it healthier —some paradox in a way — or better able to resist the kinds of bugs that might kill it, then the virus has got a fighting chance of doing that.

Skip Virgin: We need to think about certain viruses in the terms of how long they have co-evolved with the species that they infect and is there any advantage back to the host. So, I think you have made a good point certainly something that we have been thinking a lot about. It turns out that the ancestral herpes virus, if you will, that has given rise to the human herpes viruses and the mouse viruses that we studied, and a number of other viruses probably can be traced back to an ancestor that was present before the divergence between birds, reptiles, and mammals. So, these viruses have been evolving with the species that they inhabit for probably tens or even hundreds of millions of years. So, there has been a long time for the host and the virus to get used to each other and to learn to live with each other.

Chris Smith: I guess the key question is —you have shown this in animals, and is it going to work in humans? Is the same thing true for us?

Skip Virgin: That is a really great question and one that we have thought a lot about. These viruses are so common the majority of humans carry not one, but many, of these viruses for their entire lifetime; and so I think we are going to struggle proving that there is an advantage in humans, because most humans actually have one or more of these viruses as permanent passengers, if you will.

Chris Smith: Can we steal what they do though, and use it in medicine? Could we find out how these viruses are triggering this more robust and resilient immune state and then confer that on people without having to give them a herpes virus?

Skip Virgin: I think that we can definitely think about doing that, and the hormone interferon-gamma, which we believe is the key mediator of this symbiotic advantage that is being conferred by these latent viruses, that is already in use and in fact in clinical use in certain circumstances to protect people. It is a very powerful hormone. It also has some side effects. Another way to look at this is perhaps we can learn how to engineer the virus so that it can confer the advantage but does not have some of the disadvantages that these viruses do have. So that is something that we are very interested in thinking about how to do.

[RD]

Q. Are any viral infections good for our health?

How about oncolytic viruses ? ... https://en.wikipedia.org/wiki/Oncolytic_virus

[chris]

Yes, that's a very good point @RD Of course those are not usually "natural" infections and, more normally, scientists have to adapt a virus to endow it with a tropism for cancer cells, or confer additional immune-stimulating signals, to make this work. Not always though, and a story we covered recently considered the use of Zika virus for brain cancer treatment.

This is the interview with Ian Goodfellow and Harry Bulstrode about Zika virus and brain tissue and the use of the virus for killing glioblastoma cells.

[SquarishTriangle]

Do viruses exist to infect living things, or do living things exist to facilitate the existence of viruses? Are we all just the machinery for viral reproduction?

Philosophical question...

[chris]

Do viruses exist to infect living things, or do living things exist to facilitate the existence of viruses? Are we all just the machinery for viral reproduction?

Philosophical question...

There's always one...