# Is there such thing as a "good" virus? **URL:** \ **Category:** Factual Questions\ **Created:** [February 24, 2009, 3:10am UTC](https://boards.straightdope.com/t/is-there-such-thing-as-a-good-virus/487151 "2009-02-24T03:10:34Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![No\_Wikipedia\_Cites](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@No\_Wikipedia\_Cites](https://boards.straightdope.com/u/No_Wikipedia_Cites)\ **Post date:** [February 24, 2009, 3:10am UTC](https://boards.straightdope.com/t/is-there-such-thing-as-a-good-virus/487151/1 "2009-02-24T03:10:34Z")
Thinking about the virus that causes AIDS, which at least until recently was a sure death sentence, I wondered what would it benefit a virus to kill off its host. Wouldn’t it be evolutionarily superior for a virus to instead just hang around until the end of the host’s natural lifespan? But if that’s so, why don’t we ever hear of any good viruses? If a virus had any positive impact on people, it wouldn’t be cured or avoided. Yet we have only bad ones? ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [February 24, 2009, 3:28am UTC](https://boards.straightdope.com/t/is-there-such-thing-as-a-good-virus/487151/2 "2009-02-24T03:28:31Z")
Because a virus doesn’t need to be good, only good enough to pass on its genetics and create a lot more viral particles. Evolution selects for whatever is most effective at making copies of itself that are, themselves, most effective at making copies of themselves. That said, there are viruses that will normally never kill you but are effectively impossible to root out entirely. Herpes is a prime example: One case of chickenpox and you are subject to cold sores and/or shingles the rest of your life, because the virus that causes all of them never really leaves you but is effectively dormant most of the time. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 24, 2009, 3:33am UTC](https://boards.straightdope.com/t/is-there-such-thing-as-a-good-virus/487151/3 "2009-02-24T03:33:11Z")
One class of viruses, called bacteriophages, only attack bacteria. Each strain of phage only attacks a specific species of bacteriom. As such, we can use these to combat various bacterial infections; I’d call this a "good"virus if that term has any meaning. ---
**Author:** ![Cagey\_Drifter](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@Cagey\_Drifter](https://boards.straightdope.com/u/Cagey_Drifter)\ **Post date:** [February 24, 2009, 3:35am UTC](https://boards.straightdope.com/t/is-there-such-thing-as-a-good-virus/487151/4 "2009-02-24T03:35:28Z")
related thread from a couple years ago: > **[Are there any "diseases" or contagious conditions that confer benefits?](https://boards.straightdope.com/sdmb/showthread.php?t=335192)** > > All diseases\* that I know of hurt you somehow. Are there any that give you benefits? I would guess that there are some that cause some slight changes that some might describe as beneficial, but I’m sure that they are only slight. I’m really looking... ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [February 24, 2009, 4:00am UTC](https://boards.straightdope.com/t/is-there-such-thing-as-a-good-virus/487151/5 "2009-02-24T04:00:23Z")
> [@sassyfras](#): > > Thinking about the virus that causes AIDS, which at least until recently was a sure death sentence, I wondered what would it benefit a virus to kill off its host. Wouldn’t it be evolutionarily superior for a virus to instead just hang around until the end of the host’s natural lifespan? > > But if that’s so, why don’t we ever hear of any good viruses? If a virus had any positive impact on people, it wouldn’t be cured or avoided. Yet we have only bad ones? Actually, about 1-2% (some sources say up to 8%) of the human genome is of viral origin, the result of ancient infections by retroviruses (the same class of virus as HIV). Retroviruses integrate their genes into the DNA of the host. When they infect the reproductive cells, they can be passed down as endogenous retroviruses (ERVs). It is thought that such a viral infection may have aided the evolution of viviparity in mammals. From [here:](http://en.wikipedia.org/wiki/Endogenous_retrovirus) > [@](#): > > Endosymbiotic ERVs in mammals > During pregnancy in viviparous mammals (all mammals except Monotremes), ERVs are activated and produced in high quantities during the implantation of the embryo. They are currently known to act as immunodepressors, protecting the embryo from its mother’s immune system. Also viral fusion proteins apparently cause the formation of the placental syncytium[2] in order to limit the exchange of migratory cells between the developing embryo and the body of the mother (something an epithelium will not do sufficiently, as certain blood cells are specialized to be able to insert themselves between adjacent epithelial cells). The ERV is a virus similar to HIV (which causes AIDS in humans). The immunodepressive action was the initial normal behavior of the virus, similar to HIV, the fusion proteins were a way to spread the infection to other cells by simply merging them with the infected one (HIV does this too). It is believed that the ancestors of modern viviparous mammals evolved after an infection by this virus, enabling the fetus to survive the immune system of the mother. [3] > > The human genome project found several thousand ERVs classified into 24 families.[4] Some scientists have also proposed that retroviruses also played a role in the [divergence of humans ancestors from chimpanzees.](http://www.sciencedaily.com/releases/2002/08/020802075138.htm)